328 PRINCIPLES OF CHEMISTRY 



As the molecules of many elements (hydrogen, oxygen, nitrogen, 

 chlorine, bromine, sulphur at least at high temperatures) are of uni- 

 form composition, the formulae of the compounds formed by them 

 directly indicate the composition by volume. So, for example, the 

 formula HNO 3 directly shows that in the decomposition of nitric acid 

 there is obtained 1 vol. of hydrogen, 1 vol. of nitrogen, and 3 vols. 

 of oxygen. 



And since a great number of mechanical, physical, and chemical 

 properties are directly dependent on the elementary and volumetric 

 composition, and on the vapour density, the accepted system of atoms 

 and molecules gives the possibility of simplifying a number of most 

 complex relations. For instance, it may be easily demonstrated that 

 the vis viva of the molecules of all vapours and gases is alike. For 

 it is proved by mechanics that the vis viva of a moving mass =5 mv 2 , 

 where ra is the mass and v the velocity. For a molecule, m=M, or 

 the molecular weight, and the velocity of the motion of gaseous 

 molecules=a constant which we will designate by C, divided by the 

 square root of the density of the gas 25 =C/v/D, and as D=M/2, 

 the vis viva of molecules =C 2 that is, a constant for all molecules. 

 Q.E.D. The specific heat of gases (Chapter XIV.), and many other 

 of their properties, are determined by their density, and consequently 

 by their molecular weight. Gases and vapours in passing into a 

 liquid state evolve the so-called latent lieat, which also proves to be 

 in connection with the molecular weight. The observed latent heats 



* Chapter I., Note 84. 



88 The velocity of the transmission of sound through gases and vapours closely 

 bears on this. It = J EpglD (1 + a<)> where K is the ratio between the two specific heata 

 (it is approximately 1*4 for gases containing two atoms in a molecule), p the pressure 

 of the gas expressed by weight (that is, the pressure expressed by the height of a column 

 of mercury multiplied by the density of mercury), g the acceleration of gravity, D the 

 weight of a cubic measure of the gas, a = 0-00867, and t the temperature. Hence, if K 

 be known, and as D can be found from the composition of a gas, we can calculate the 

 velocity of the transmission of sound in that gas. Or if this velocity be known, we can 

 find K. The relative velocities of sound in two gases can be easily determined (Kundt). 



If a horizontal glass tube (about 1 metre long and closed at both ends) be full of ^a 

 gas, and be firmly fixed at its middle point, then it is easy to bring the tube and gas into 

 a state of vibration, by rubbing it from centre to end with a damp cloth. The vibration of 

 the gas is easily rendered visible, if the interior of the tube be dusted with lycopodiura 

 (the yellow powder-dust or spores of the lycopodium plant is often employed in medicine), 

 before the gas is introduced and the tube fused up. The fine lycopodium powder arranges 

 itself in patches, whose number depends on the velocity of sound in the gas. If there 

 be 10 patches, then the velocity of sound in the gas is ten times slower than in glass. It 

 is evident that this is an easy method of comparing the velocity of sound in gases. II 

 lias been demonstrated by experiment that the velocity of sound in oxygen is four timed 

 less than in hydrogen, and the square roots of the densities and molecular weights of 

 hydrogftn and oxygen stand in this ratio. 



