THE SECOND STEP IN CHEMISTRY.

CHAPTER I.

PROPERTIES OF MATTER.

Exercises.

Specific gravity of gases and vapours, 1. Expansion of matter by heat, 5. Correction of gases for temperature, 12. Exercises. Compressibility of matter, 16. Elasticity of matter, 25. Correction Correction of gases for pressure, 42. Exercises. Liquefaction, 43. Vaporization, 45. Correction of gases for the tension of aqueous vapour. Exercises.

1. SPECIFIC GRAVITY OF GASES AND VAPOURS.-Gases and vapours differ in their densities, or specific weights. 2. Atmospheric air, at 60° F., and the barometer standing at 30 inches, is employed as the standard of comparison for gases and vapours. One hundred cubic inches of air weigh, according to the latest researches by Regnault, 30.935 grains at this temperature and pressure. Air is therefore about 814 times lighter than water, as 100 cubic inches of water weigh 25246'0 grains.

3. If the specific gravity of a gas or vapour be known, the absolute weight of a given volume of it can be determined. To accomplish this, we have simply to multiply the weight of an equal volume of air by the specific weight of the gas or vapour; the product will be the weight of the volume, at the standard temperature and pressure of the gas or vapour.

Example. What is the weight of 100 cubic inches of hydrogen, its specific gravity being '0694?

30-935 × 0694 = 2·147 grains weight of 100 cub. in. of H. 4. If it be desired to find the volume of a given weight

The standard temperature and pressure adopted on the Continent differs from that employed in England. The temperature is 0° C. = 32° F. ; the pressure is 760 millimetres = 29-922 inches.

B

of any gas or vapour, the weight of some volume (say a cubic inch) of the gas or vapour must first be ascertained by the preceding rule; then the given weight must be divided by the weight of the cubic inch; the quotient will be the volume in cubic inches of the given weight of the gas, at the standard temperature and pressure.

Example.-What is the volume of 2·147 grains of hydrogen, its sp. gr. being 0·0694 P

1. What is the weight of a cubic inch of oxygen, its sp. gr. being 1.1057 ?

2. What is the weight of a cubic inch of nitrogen, its sp. gr. being 0.9713 P

3. What is the weight of a cubic inch of carbonic acid, its sp. gr. being 1·529 ?

4. What is the weight of a cubic inch of gaseous ammonia (NH3), its sp. gr. being 0·59?

5. What is the volume of 54 grains of chlorine, its sp. gr. being 2:44?

6. What is the volume of 45 grains of carbonic oxide, its sp. gr. being 0.967 ?

7. How many cubic inches of oxygen would be obtained from 100 grains of chlorate of potash?

Oxygen gas is frequently prepared by igniting peroxide of manganese. The following decomposition occurs:3 Mn 0,= Mn, 0, +20.

8. How many cubic inches of oxygen would be obtained from 400 grains of a manganese ore containing 70 per cent. of the peroxide, the equivalent of manganese being 27.6 ?

Gunpowder consists of a mixture of carbon, sulphur, and nitrate of potash. The gunpowder which is most powerful as a propelling agent, is found to be that which corresponds most nearly in composition to the formula,— KO, NO, + 3C + S.

The theoretical decomposition of a powder of this description would be represented by the equation,

KO, NO, +3C+S=KS+3CO, + N.* In practice, it is found that small quantities of many other products are invariably formed besides CO,, N, and KSt; among which may be mentioned carbonic oxide, hydrosulphuric acid, bisulphide of carbon vapour, carbonate of potash, cyanide and sulpho-cyanide of potassium, and aqueous vapour.

9. How many cubic inches of gas would be produced, at the standard temperature and pressure, from 1.000 grains of gunpowder, having the above composition, according to the theoretical decomposition?

5. EXPANSION OF MATTER BY HEAT.-All solid, liquid, and gaseous substances expand when they are heated, and contract when they are cooled; and the rate and degree of expansion of each substance is always the same, no matter how often it may be heated, if its temperature be the same at the commencement of each experiment, and the same amount of heat be added. For example,if a substance at the temperature of 40° F. be heated up to 100° F., it will always expand to the same bulk when raised from 40° to 100° F., and it will always contract to the same bulk when cooled down again to 40° F.‡

6. The same amount of heat affects matter in its three states very unequally. It produces the greatest expansion in gases, and the least in solids, because there is no cohesiveness between the particles of matter in the gaseous state; the heat has, therefore, no opposing force to overcome; and as the cohesive force is less in liquids than in solids, a less amount of heat is neutralized in overcoming this opposing force, and, therefore, for the same amount

*The gases disengaged in the combustion of this powder would occupy, at 32 F., a volume 329 times that occupied by the powder; but at the moment of ignition, the volume of gas is 2,000 times that of the powder, on account of the enormous heat. The temperature evolved by the combustion of gunpowder has been found sufficiently intense to fuse gold and other metals. It is estimated at 2192° F.

+ The sulphide of potassium is, to a great extent, volatilized by the heat resulting from the explosion of the powder, and the highly heated vapour ignites as it passes into the air, becoming converted into sulphate of potash. The white smoke observed on the ignition of powder is produced in this way, * Lead is an exception to the rule; "it is so soft, that its particles slide over each other in the act of expansion, and do not return to their original position. For example,—a leaden pipe used for conveying steam permanently lengthens some inches in a short time; and the leaden flooring of a sink which often receives hot water, becomes, in the course of time, thrown up into ridges and puckers."-Graham.

of heat, a greater expansion takes place in liquids than in solids.

7. All gases, on account of the absence of any cohesive. ness between their particles, expand alike,* the pressure being the same, for equal additions of heat; and contract alike for equal subtractions of heat; but the different liquids and solids expand and contract unequally, for equal additions and subtractions of heat.

8. Having stated that all gases expand and contract alike, on account of the absence of any cohesiveness between their particles, the following law, "that the rate of expansion for all gases is equal and uniform at all degrees of heat," + might even be inferred.

9. Gases expand rather more than one-third their volume on being raised from the temperature at which ice melts to that at which water boils, at the ordinary atmospheric pressure; one volume of any gas, at the temperature of melting ice, expands to 1.36651 volumes, if its

All that is stated in this and the adjoining pages, with respect to gases, applies also to vapours, at some distance above their points of condensation. This law was discovered by Gay Lussac and Dalton.

It is necessary to observe, before proceeding further, that this uniform dilatation of all gases is not absolutely true. The more condensable gases display irregularities, especially near their points of solidification and lique. faction. Regnault and Magnus have published independent and elaborate investigations on the expansion which various gases undergo by the application of heat. According to their experiments, the coefficient of expansion is not rigidly uniform for all gases, the expansion being greatest for those which are most readily condensable; whilst for the gases which have resisted all efforts to liquefy them, scarcely any appreciable differences are observed. Although it is not, therefore, rigorously true that all gases, and likewise vapours, at some distances above their points of condensation, expand alike, yet it is sufficiently accurate for all the requirements of chemistry. The following table contains a summary of Regnault and Magnus's experimental results.

temperature be raised to that of boiling water. If, then, the interval from the melting point of ice to the boiling point of water were divided into 100 degrees, any gas which measured one volume at the melting point of ice would increase 003665 in bulk for each degree it inereased in temperature. In France, and on the Continent generally, the scale of the thermometer is divided in this way, it is, therefore, called the Centigrade scale; the temperature at which ice melts is reckoned 0°, and the temperature at which water boils at the standard pressure is represented as 100°; therefore, any gas which measures one volume at 0° of the Centigrade scale will measure 1003665 in bulk at 1° C., 1.00733 at 2° C., and so on: for every degree it increases or decreases in temperature by the Centigrade scale, it increases or decreases in bulk 003665.

10. In England and America, Fahrenheit's scale is principally employed; the interval between the melting point of ice and the boiling point of water, according to this scale, is divided into 180 degrees; 32° on this scale is the temperature at which ice melts, 212° that at which water boils at the standard pressure. As one volume of gas at 32° F. expands to 1.3665 at 212° F., it results that for each degree Fahrenheit that the gas increases in temperature it must increase in bulk 00204, for

and as one volume of gas increases '00204 in bulk for each degree Fahrenheit, 490 volumes of gas must increase one volume for each degree F. that it increases in temperature.

11. As gases expand equally at all degrees of heat, it is not necessary, when taking the specific gravity of a gas, to conduct the operation at a particular temperature, as we are obliged to do when taking the specific gravity of solids or liquids; it is only necessary to note the temperature of the gas at the moment the experiment is made; we are then able to calculate the volume it would occupy at the standard temperature. It is also necessary to know how to compute the volume a gas would occupy at some other temperature than that at which it is measured, for other purposes besides determining the specific gravities. We will therefore explain how the calculations are made, and give a few exercises which the student must perform.