436 Drs. Ramsay and Young on 



expression for Gay-Lussac's law is v= c(l+a£), or, if the 

 absolute temperature-scale be employed, v = cT. 



As a deduction from these laws, it follows that if the volume 

 of unit mass of a gas, supposed to follow them rigorously, be 

 kept constant, the pressure varies directly as the absolute 

 temperature ; or p = cT. 



Now, so long as the volume of unit mass of a gas is kept 

 constant, the average distance of its molecules from one an- 

 other will remain constant ; and it is a fair assumption that 

 the attraction of the molecules for each other will not vary. 

 It may, of course, be the case that the effect of a rise of tem- 

 perature on any individual molecule is to alter its actual 

 volume ; but of this we know nothing ; and, in default of 

 knowledge, it has been assumed by us that no such alteration 

 takes place. If these assumptions are correct, it follows 

 that the temperature and pressure of gases — and indeed the 

 same assumptions may be extended to liquids — should then 

 bear a simple relation to each other. We have obtained ex- 

 perimental proof of a convincing nature that this is the case ; 

 and in a preliminary note to the Royal Society, read on 

 January 6, we promised such a proof. This proof is the sub- 

 ject of the present paper ; and we must ask for indulgence 

 in quoting a large array of figures, some of which have 

 already been published, on the ground that such an important 

 generalization requires as much experimental evidence as can 

 be brought to bear on it. 



The relation between the pressures and temperatures of a 

 liquid or a gas at constant volume is expressed by the equation 



p = bT-a; 

 where p is the pressure in millimetres, T the absolute tempera- 

 ture, and b and a constants. The values of these constants 

 depend on the nature of the substance and on the volume. It 

 follows from this, that if a diagram be constructed to express 

 the relations of pressure, temperature, and volume of liquids 

 and gases, where pressure and temperature form the ordinates 

 and abscissas, the lines of equal volume are straight*. 



We have proved this to be the case for ethyl oxide (ether) 

 between the temperatures 100° and 280°, and for volumes 

 varying from 1*85 cubic centim. per gram to 300 cubic centim. 

 per gram. This proof we now proceed to give. 



The data for the calculations are at present in the press, and 

 will shortly appear in the Philosophical Transactions for 1886, 

 p. 10. A diagram (which will accompany that memoir) was 

 constructed with the greatest care, showing isothermal lines, 



* Amagat ( Comptes Rcndus, xciv. p. 847) lias stated a similar relation 

 for gases; his data are, however, imperfect, and he expressly states that 

 the law does not apply to liquids. 



