614 THE PRINCIPLES OF SCIENCE. [chap. 



any abrupt change whatever. As the pressure is greater 

 the abruptness of the change from liquid to gas gradually 

 decreases, and finally vanishes. Similar phenomena or an 

 approximation to them have been observed in other liquids, 

 and there is little doubt that we may make a wide genera- 

 lisation, and assert that, under adequate pressure, every 

 liquid might be made to pass into a gas without breach of 

 contiuiiity.^ The liquid state, moreover, is considered by 

 Professor Andrews to be but an intermediate step between 

 the solid and gaseous conditions. There are various in- 

 dications that the process of melting is not perfectly abrupt ; 

 and could experiments be made under adequate pressures, 

 it is believed that every solid could be made to pass by in- 

 sensible degrees into the state of liquid, and subsequently 

 into that of gas. 



These discoveries appear to open the way to most im- 

 portant and fundamental generalisations, but it is probable 

 that in many other cases phenomena now regarded as dis- 

 crete may be shown to be different degrees of the same 

 process. Graham "was of opinion that chemical affinity 

 differs but in degree from the ordinary attraction which 

 holds different particles of a body together. He found that 

 sulphuric acid continued to evolve heat when mixed even 

 with the fiftieth equivalent of water, so that there seemed 

 to be no distinct limit to chemical affinity. He concludes, 

 " There is reason to believe that chemical affinity passes 

 in its lowest degree into the attraction of aggregation." ^ 



The atomic theory is well established, but its limits are 

 not marked out. As Grove points out, we may by 

 selecting sufficiently liigh multipliers express any combi- 

 nation or mixture of elements in terms of their equivalent 

 weights.^ Sir W. Thomson has suggested that the power 

 which vegetable fibre, oatmeal, and other substances possess 

 of attracting and condensing aqueous vapour is probably 

 continuous, or, in fact, identical with capillary attraction, 

 which is capable of interfering with the pressure of aqueous 

 vapour and aiding its condensation.* There are many cases 

 of so-called catalytic or surface action, such as the extra- 



' Nahire, vol. ii. p. 278. 



'^ Journal of tlie ChemicnJ Society, vol. viii. p. 51. 

 3 ('orri'lation of Physical Forces, 3rd edit. p. 184. 

 ■• Philuso^hicai Magazine, 4tk Series, vol, xlii. p. 43 1 . 



