not be omitted, and that the best proportions for the coating are, one part of common pipe-clay to three parts of fine sand, which are to be well kneaded together, and reduced to such a state of tenacity, that the lute will readily adhere to the tube, and its different parts will unite without forming a visible seam. The black flux, which is mentioned by Mr. Murray, is often not to be procured, but powdered charcoal answers the same purpose. With respect to the quantity of the inflammable material, I think that considerably less than this author advises is more convenient, for when too much is employed, it requires a greater degree of heat, and a much longer application of it. In Aikins's dictionary we are directed to add "a little powdered charcoal." I believe that the effect will be best produced by using about equal bulks of the arsenic and charcoal. In order to get the materials into the tube, a wooden rammer must be formed, by which they may be pressed down to the closed end. A tube 1-4th of an inch in diameter, coated with a thin covering of clay and sand, requires to be kept at least 1-4th of an hour in a well burnt coal fire, in order to have the full effect produced. This method of detecting arsenic has been esteemed the most decisive, but it is by no means the most minute of the tests; when less than 3-4ths of a grain was employed, I could not say that the metallic coating was clearly perceptible. From a remark that is made by Dr. Black, it would appear, that one grain was the smallest quantity which he thought capable of detection by this process.*

The property which arsenic possesses of uniting with copper, and forming a white compound, is the next method that has been employed to detect the presence of the former substance. To produce the effect, we are directed to place the arsenic, mixed with some carbonaceous matter, between two plates of polished copper, and having bound them together by a wire, to expose them for some time to a red heat, when the part of the copper that is in contact with the arsenic, will be found to have received a permanent white stain. The Messrs. Aikins direct the suspected powder to be mixed with a little charcoal, while Mr. Murray recommends it to be made into a paste with the black flux and oil. I should here, for the same

* Black's Lectures, v. ii. p. 430.

reason as before, prefer the charcoal to the black flux, but I think the oil is certainly a useful addition. When the dry charcoal is employed, the powder is in danger of escaping from between the plates, unless they be very closely pressed together, and when this is the case, a considerable part of it will be found unconsumed after they have been heated. With respect to the proportion of the ingredients to be employed, I have found the process to succeed the best, when to one grain of the arsenic we add about one half grain of powdered charcoal, and about two drops of oil. This property of whitening copper is justly regarded as one of the most decisive tests of the presence of arsenic; but there are some circumstances attending it, which may deserve to be pointed out to the inexperienced. It may be necessary, in the first place, to describe the phenomena that take place when copper is heated according to the process that is described above, but without the addition of the arsenic. Two copper disks, of nearly an inch and a half in diameter, scoured bright with sand, had one grain of powdered charcoal, made into a paste with oil, placed between them; they were bound together with an iron wire, and then kept red hot for ten minutes. When they were withdrawn from the fire, the metal was found to have lost its former appearance, and to have acquired the dull white colour of lead or zinc. The insides of the disks were found to present the same whitish appearance, except on the spot where the charcoal was placed, a small part of which still remained unconsumed. As the disks cooled, the whitish matter which covered them began to separate, and fly off with some force, in the form of small scales, leaving a clean surface of the proper copper colour. In a few minutes, the greatest part of the scales were removed, except on the inside, near where the charcoal had been placed, and here the disks still retained their gray colour. The charcoal was rubbed off, and the surface below it was found smooth and polished; it had acquired a light colour, resembling that of brass, and near the centre there was a small spot, which approached to a steel gray. This appearance still continued, after it had been rubbed with fine sand. The above description of the changes which the disks experience, by being heated with a part of their surface in contact with an inflammable body, will probably impress the society with the same idea,

that, I confess, it gave to myself, that if I had performed this experiment upon a substance which was suspected to contain arsenic, and had not been aware of the appearance that I was to meet with, I should have conceived that I had detected its presence. Upon repeating the process, in precisely a similar manner, except that one grain of arsenic was added to the charcoal, the oxydation of the copper took place as before, and a small part of the charcoal remained unconsumed; but upon rubbing it the white stain was perfectly visible. However, when these disks were compared with those in which the former experiment had been made, the difference between them seemed more in degree than in kind, so that I should not choose to decide upon the presence of arsenic as indicated by this test, unless the result was more obvious than it must be expected to be, when it exists only in small quantity. It may be proper to observe, that copper whitened in this manner by arsenic, is very subject to tarnish; in three days I could with difficulty distinguish which of the disks had been employed in these two experiments.

With respect to the 5th method that has been pointed out for detecting arsenic, by exposing it to a high temperature, and observing the white smoke and peculiar odour which arise from it in this situation, I shall not think it necessary to make many remarks. It is certainly not much to be depended upon; unless the arsenic is in considerable quantity, the effects are not sufficiently perceptible; and if it be mixed with either an animal or a vegetable substance, the smoke and smell arising from these bodies, when heated, will altogether prevent us from detecting the peculiar properties of the arsenic. When a quantity of arsenic is mixed with an equal weight of flour, and placed upon iron at a low red heat, so as not to cause the flour to inflame, the suffocating smoke that arises from the flour can alone be perceived; nor is it possible to discover that any thing has been mixed with it. With respect to the degree of heat, at which the characteristic odour of arsenic is most readily discovered, by having a bar of iron heated red hot at one end, and becoming gradually cooler towards the other extremity, I thought that the effect was the most perceptible in that part which was just visibly red in bright daylight.

After this review of the different processes for detecting

arsenic, it remains for us to determine on which of them the greatest confidence may be placed. I have no hesitation in giving it as my opinion, that the most convenient, the most delicate, and the most decisive process, is the one in which the green precipitate is formed by the addition of the sulphate of copper. The arsenic may be detected with certainty in a very minute quantity, as well in a fluid as in a solid state; no apparatus is requisite, nor is there any skill required on the part of the operator. When we are called upon, in a judicial investigation, to ascertain whether arsenic has been employed, we are generally obliged to perform our experiments on a small quantity of the substance which may be accidentally left in the cup or bottle, or to search for it among the contents of the stomach. In this latter case, particularly, it is only in minute quantity that we can expect to find it; and in order to separate it from the mass in which it is contained, we must have recourse to the solvent power of hot water.

There are two precautions which I should strongly recommend to every person who may be called upon to search for arsenic in a suspected substance. The first is, that in everyexperiment which is performed upon the unknown substance, a corresponding one should be made with ingredients of a known composition. If, for example, the test of the sulphate of copper be applied to a suspected fluid, the operator should at the same time add the potash and sulphate of copper to a portion of pure water, and afterwards he should repeat the process with the potash, sulphate of copper, and a given quantity of an arsenical solution. The effect of the reagents will thus become familiar to him both with and without the arsenic, and he will then be enabled, with much more confidence, to state his opinion respecting the fluid under consideration. The second recommendation which I shall beg leave to offer, is, that the examination be never entered upon, until the substances that are intended to be employed as tests, and the necessary apparatus, are all in readiness, and all in a perfect state. In experiments of this kind, there should be no allowances made; and as it often happens that the quantity of the matter to be operated upon is small, none of it ought to be wasted upon imperfect trials.

Liverpool, February 10, 1809.

From Philosophical Transactions for 1810. Part I.

I AM aware that the remarks which I have to offer on the present occasion, may be thought to bear too little direct relation to each other for insertion in the same lecture; yet any observation respecting the mode of action of voluntary muscles, and every inquiry into the causes which derange, and into the means of assisting the action of the heart and bloodvessels, must be allowed to promote the design of Dr. Croone, who instituted these annual disquisitions. And it has always appeared to be one great advantage attending the labours of this society, that it favours the production of any original knowledge, however small, in a detached form; and enables a writer to say all that he knows upon a particular subject, without inducing him to aim at the importance of a long dissertation. I shall therefore make no apology for dividing the following lecture into three distinct parts.

In the first of which I shall treat of the duration of voluntary action.

In the second I shall attempt to investigate the origin of seasickness, as arising from a simple mechanical cause deranging the circulation of the blood.

In the third, I shall endeavour to explain the advantage derived from riding, and other modes of gestation, in assisting the health under various circumstances, in preference to every species of actual exertion.

PARTI, ON THE DURATION OF MUSCULAR ACTION.

The necessity of occasional intermissions from a series of laborious exertions, is within the experience of every one; the fatigue of continuing the effort of any one voluntary muscle without intermission even for a few minutes is also sufficiently known; but there is a third view of the duration of muscular action which appears to have escaped the notice of physiologists. For I believe it has not hitherto been observed that each effort, apparently single, consists in reality of a great number of contractions repeated at extremely short intervals; so short