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Platino-cyanides.- Just as platinous chloride combines with metallic chlorides to form chloro-platinates, so platinous cyanide, Pt(CN),, unites with other cyanides, forming similarly constituted double compounds, known as platino-cyanides.*
Potassium platino-cyanide, K.Pt(CN)ų, or 2KCN,Pt(CN)2, is formed when spongy platinum is dissolved in boiling potassium cyanide. The platino-cyanides may be regarded as the salts of platino-cyanic acid, H,Pt(CN)! Both the acid and the salts are characterised by the wonderful play of colours they exhibit when viewed in different lights, and by forming different coloured crystals with varying quantities of water of crystallisation (see page 217).
Sulphides of Platinum.--Platinous sulphide, Pts, and platinic sulphide, PtS,, are obtained as amorphous black powders by the action of sulphuretted hydrogen upon the respective chlorides.
Oxysalts of Platinum.-Few well - defined single salts of platinum with oxyacids are known. This element, however, exhibits a great tendency to form complex double salts. One such series of compounds is seen in the platino-nitrites, which may be regarded as the salts of platino-nitrous acid, H.,Pt(NOX)
These salts are remarkable, in that the platinum they contain cannot be detected by the ordinary tests for that metal ; just as the iron present in ferro-cyanides is not detected by the ordinary reagents used in testing for that metal.
Ammoniacal Platinum Bases, or Platinamines.
Like cobalt, platinum forms a large number of basic compounds with ammonia, many of which are of extremely complex composition. The first of these to be discovered was a bright green salt, obtained by the action of ammonia upon platinous chloride, having the composition PtCl., 2N Hz, or PI(NH3),Cly, and known as the green salt of Magnus. Many of the platinamines exhibit isomerism ; thus, a compound known as the chloride of Reiset's second base is a yellow crystalline salt having the same composition as Magnus's green salt. Twelve distinct series of ammoniacal platinum compounds are known, four of which are derived from platinous and the remainder from platinic salts; the former are termed platoso ammonia compounds, while the latter are distinguished as the platino compounds.†
* The name Cyano-platinites might with advantage be applied to these compounds.
+ For detailed descriptions of these compounds, the student is referred to larger works on chemistry; and on the constitution of these, and metallammonium compounds generally, the article by Werner, in the Zeitschrift für Anorganische Chemie, 1893, vol. iii. p. 267, may be consulted.
RADIUM, AND RADIOACTIVE ELEMENTS
As far back as the year 1896, Becquerel discovered that the element uranium and its salts possess the remarkable property of emitting rays somewhat similar in character to the now familiar Röntgen or “X" rays; resembling these rays in their penetrating power, their photographic action, and their action upon electrified gases. These peculiar rays were distinguished from the Röntgen rays by being called the “uranium," or the “ Becquerel " rays. Somewhat later it was found that the element thorium and its compounds were likewise possessed of the property of emitting rays, which, while differing from both the “X” and the “ uranium" rays in some respects, closely resembled them in others. To denote this property, the term radioactivity has been coined, and substances possessing the property are said to be radioactive bodies.
In 1898 it was announced that M. and Mme. Curie had discovered a new radioactive substance contained in pitchblende, a mineral consisting essentially of uranium oxide. From researches already made, it had been shown that the radioactivity of uranium compounds is roughly proportional to the amount of the metal present, but it was found that in the case of certain specimens of pitchblende this was not the case, but that the radioactivity was greatly in excess of that calculated from the percentage of uranium in the mineral. This fact suggested the presence of some new substance of superior radioactivity to that possessed by uranium. It was found in the ordinary process of separation of the metals by precipitation from an acid solution by sulphuretted hydrogen, that this new active substance was thrown down along with the sulphides, and finally was separated from the copper and arsenic, &c., and remained associated with the bismuth. No isolation of the new substance was effected, but from its greatly superior radioactivity the discoverers concluded that there was sufficient evidence of the presence of a new element to warrant them in giving it a name. They therefore called it polonium, from the country from which the pitchblende was obtained. * (Compt. rend. 127, p. 175.)
Following up their investigations, the same workers very shortly afterwards discovered in the same mineral another radioactive body of suill far greater activity. This new substance, they found, is not precipitated by either sulphuretted hydrogen, ammonium sulphide, or ammonia, but is associated
* Although the name polonium is still met with in the literature of the subject, no further evidence has been produced in proof of the existence of a new element corresponding to the name. The name is used rather to denote the radioactivity which appears to be associated with the element bismuth.
with and accompanies barium in the various chemical reactions the latter element undergoes. Thus, when barium sulphate or carbonate is precipitated from a solution of the chloride, the precipitated barium compound is accompanied by the radioactive material; or when the chloride itself is precipitated either by strong hydrochloric acid or by alcohol, the "active" substance is thrown down along with it.
By the careful fractional precipitation of the chloride with alcohol it was found possible to gra lually concentrate the radioactive substance in the barium chloride, and in this way a product was obtained possessing a radioactivity 900 times greater than that of uranium. In view of the intensity of its “activity," the discoverers gave the name radium to the new element which they believed to be present, although in almost infinitely minute quantities. (Compt. rend. 127, p. 1215.)
The spectrum exhibited by this "active" barium chloride also confirmed the presence of a new element, for besides the lines belonging to barium it contained a well-defined line which had never previously been observed in the spectra of any of the known elements. • Determinations of the atomic weight of the metal (barium) in the specimens of barium chloride which contained the radioactive element to an extent sufficient to show an “activity" 900 times greater than that of uranium, gave values practically the same as those of ordinary barium, namely 137.4. That is to say, the actual amount of radium which gave rise to so high 'activity" in the barium chloride was too small to influence the atomic weight determination. When, however, the concentration of the radium chloride in the barium chloride was considerably increased by a continuation of the fractionating process, the atomic weight of the metal was found gradually to rise. Thus, when the intensity of the radioactivity reached 3000 times that of uranium, the atomic weight of the “barium " rose to 140; while with a concentration representing a radioactivity 7500 times that of uranium, the atomic weight of the metal present was found to be 145.8. From these determinations it was evident that radium would be found to be an element of very high atomic weight, and in the course of time when it became possible to obtain small quantities of radium compounds - such as the chloride and bromide--in a state of comparative purity, this was found to be the case. The latest determinations (Curie, Compt, rend. 1902, 135, p. 161) have assigned the number 225 as the atomic weight of this new element-a value which the discoverers regard as being correct to one unit. From purely spectroscopic considerations, however, Runge and Precht (Ast. Journ., April 1903) calculate the atomic weight of radium to be 238.
The element radium appears to resemble barium in its chemical relations. Thus the sulphate is insoluble in water and in acids; the carbonate is insoluble in water, and the chloride is precipitated by both strong hydrochloric acid and alcohol.
As seen in the Bunsen fame, the strongest and most permanent line produced by radium bromide is the blue line 4826.
The metal itself has not yet been isolated, * and in view of the extreme
* In the literature of the subject the name radium is constantly employed when in reality a radium salt is intended.
minuteness of the quantities of this element which occur in the mineral pitchblende this need be no cause for surprise. Not only is the amount of radium present in this mineral too small to be detected by any chemical test, but the spectroscope itself does not afford a sufficiently delicate means for its detection; and it is not until the quantity naturally present has been greatly concentrated by the process already described, that the characteristic spectrum even begins to make its appearance.
The chief interest attaching to this new element is associated with the strange property it possesses in such a high degree of emitting “radiations." Radium bromide, for example, is self-luminous in the dark; the rays it emits are capable of acting upon a photographic plate, much as the Röntgen rays affect it. They cause phosphorescence upon a screen of barium platinocyanide, and produce radiographic effects similar to those given by the "X" rays. They are capable of penetrating metals, and will discharge an electroscope not only through considerable intervals of space, but also through screens of various materials.
Most mysterious of all, they appear to possess the power of exciting a temporary radioactivity in other substances otherwise inactive. Thus, if a solution of a radium salt and some distilled water are placed in separate dishes in a perfectly closed space, radioactivity is communicated to the water. The water, however, gradually loses this power even in a closed space, while it rapidly loses it if exposed. It has been found also that the intensity of this "induced " radioactivity is the same for all substances, under the same conditions, irrespective of their chemical nature.
Concerning the nature and the cause of the radiations emitted by radium and the other two well-defined radioactive elements uranium and thorium,* a large amount of experimental work has been done, and much speculation put forward. As the outcome of the former it has been established that at least four distinct, and to some extent separable emissions, may go to make up what is included in the term “radiations." These are distinguished as a, b, and a rays, and “radioactive emanation."
1. The a Rays.-- These rays are very easily absorbed by thin layers of matter. Thus, a thickness of aluminium 0.0005 cm, reduces their intensity to one-half. To them is mainly attributable the property of causing the ionisation of a gas, whereby its electrical conductivity is increased. They are deviated by a very strong magnetic field, the deviation being in the opposite direction to that exhibited by " cathode" rays. These a rays are not waves like ordinary light rays, but consist of actual matter, which is being projected at an enormous velocity, and is highly charged with positive electricity. They are described as a “flight of material particles," having a mass of the same order as the atoms of hydrogen,t and travelling with a velocity about one-tenth that of light. These particles carry with them a relatively enor
* Polonium, and the still more recent actinium, are at present too undefined to be included as elements. ti.e., the ratio of the charge of the carrier to its mass is = 6X 109.
I That is, about 2.5x 109 cms. per sec. (Rutherford and Soddy, Phil, Mag., Feb. 1903).