Dr. Tliomas Andrews : The Great Chemist 131 



an account of many experiments, the bearing of which was not 

 immediately perceiA-ed. In comparing the behaviour of CO2 at 

 various temperatures and pressures, he deduced several la\vs 

 connecting the different isothermals, but it is impossible to give 

 any detailed account of the investigations. It is of interest, 

 however, to consider one of the suggested laws and to connect it 

 with some others offered by such men as Kankiue and Van der 

 Waal. His experiments showed that, for a great part of the 

 range, the isothermal curves of the vapour approximately answered 

 the equation C = V {\ - Tp), where V is the volume, p the 

 pressure, and C a constant. A perfect gas being represented by 

 Fp = constant, this equation of Andrews showed the departure 

 of the vapour from the ideal. If two such equations on the same 



C C 



isothermal curve, in the form ^, = 1 - Fi Pi and -—=\ - V^ P^, 



C C 

 be subtracted, the result is ^^ - ^^ = F2 Po - F] A- If the 



equation be assumed true for the gas from a very low to a 

 moderate pressure, then we may take Po as very low, and 



consequently -p^ may be supposed to vanish as Po becomes 



Q 



indefinitely small, leaving the form y^== R - VP, where R is the 



ideal VP when P becomes very small. This then appears to be 

 the true form of Andrews' equation, and if the proper corrections 

 are made for the presence of air, &c., it represents the true facts 

 within reasonable limits of pressure. It may obviously be thrown 



into the form R= F(P + ^J. For very high pressures this is 



obviously untrue. It becomes necessary to have a factor which 

 vanishes when P is infinite, and this is found by writing (F-a) 

 instead of F as the factor outside the bracket, when a must be 

 the limit of volume of the gas as P becomes very great, This 



then gives Van der Waals' equation (F-a)(P+^2)- With 



