THE MOLECULAR WEIGHT OF LIQUIDS. 275 



The extreme variation for the fourteen substances which 

 have been most carefully investigated is between 2*020 for 

 ethyl formate and 2-248 for methyl isobutyrate. On the 

 assumption that this is the extreme divergence, there may 

 be a positive or negative error of 5 per cent, caused by 

 assuming the mean value to be 2*121. 



In conclusion a short review of the evidence which 

 proves in a general way that certain liquids, and especially 

 the fatty acids, alcohols and water, possess complex mole- 

 cules may not be out of place. 



Guye has shown that the critical co-efficient of a liquid, 

 that is, the quotient obtained on dividing the absolute 

 critical temperature by the critical pressure, should, when 

 multiplied by the constant 1*8, be equal to the molecular 

 refraction. 



Whilst the majority of substances examined by Guye 

 appear to consist of simple molecular groups at their critical 

 points, water, methyl alcohol and acetic acid yield numbers 

 which point to association, inasmuch as the constant, instead 

 of having its usual value i*8, has decreased to about n. 



Young- and Thomas have shown that the densities of 

 most liquids at their critical points may be found by multi- 

 plying their theoretical densities by a number approximately 

 equal to 3 '85. Methyl, ethyl and propyl alcohols and acetic 

 acid are, however, exceptions to this rule, inasmuch as the 

 factor varies between four and five. This fact again 

 points to the presence of complex molecules at the critical 

 temperature. 



In determining the true volume of water and of the 

 above liquids at their critical points by the method suggested 

 by Cailletet and Matthias, the same authors again find 

 evidence of association in the liquid state. 



When the numbers representing heats of vaporisation 

 of a compound increase to a maximum and then diminish, 

 Guye has shown from Ramsay and Young's work that the 

 compound contains complex molecules in the liquid state. 

 This was found to be the case with ethyl alcohol and acetic 

 acid. The fact that the vapour pressure curve of a liquid 

 cuts those of undeniably simple substances, such as benzene, 



