MOLECULES AND ATOMS 321 



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

 form composition, therefore, the formula; of the compounds formed by 

 them, directly indicates the composition by volume. So, for example, 

 thr 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 as a great number of mechanical, physical, and chemical 

 properties are directly dependent on the elementary and volumetric 

 composition, and on the vapour density ; so 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=^ wi? 2 , 

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

 molecular weight, and the velocity of the movement of gaseous 

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

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

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

 molecules. Q.E.D. 26 The specific heat of gases (as we shall afterwards 

 .see), 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 heat, 

 which also proves to be in connection with the molecular weight. The 

 observed latent heats ofj carbon bisulphide, CS 2 =90, of ether. 



25 Chap. I. Note 34. 



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

 bears on this. It = v' Kpg/D (1 + at) where K is the ratio between the two specific 

 heats (it is approximately T4 for gases containing 2 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, o = 0'00367, 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 determined with peculiar 

 ease (Kundt). 



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

 gas, and be firmly fixed in the middle, then it is easy to bring the tube and gas into a 

 stair 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 lycopodium 

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

 In -fore- the gas is introduced and the tube fused up. The fine lycopodium powder forms 

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

 be 10 figures, 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. It 

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

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

 hydrogen and oxygen stand in this ratio. 

 VOL. I. 



UNIVRSITY 



