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SCIENCE. 



[N. S. Vol. XVI. No. 406. 



tions were proceeding, Regnault and 

 Magnus had completed their refined in- 

 vestigations on the laws of Boyle and Gay- 

 Lussac. A very important series of ex- 

 periments was made by Joule and Kelvin 

 'On the Thermal Effects of Fluids in 

 Motion' about 1862, in which the ther- 

 mometrical effects of passing gases under 

 compression through porous plugs fur- 

 nished important data for the study of the 

 mutual action of the gas molecules. No 

 one, however, had attempted to make a 

 complete study of a liquefiable gas through- 

 out wide ranges of temperature. This 

 was accomplished by Andrews in 1869, and 

 his Bakerian Lecture 'On the Continuity 

 of the Gaseous and Liquid States of Mat- 

 ter' will always be regarded as an epoch- 

 making investigation. During the course 

 of this research Andrews observed that 

 liquid carbonic acid raised to a temperature 

 of 31° C. lost the sharp concave surface of 

 demarcation between the liquid and the 

 gas, the space being now occupied by a 

 homogeneous fluid which exhibited, when 

 the pressure was suddenly diminished or 

 the temperature slightly lowered, a peculiar 

 appearance of moving or flickering striae, 

 due to great local alterations of density. 

 At temperatures above 31° G. the sepai-a- 

 tion into two distinct kinds of matter 

 could not be effected even when the pressure 

 reached 400 atmospheres. This limiting 

 temperature of the change of state from 

 gas to liquid Andrews called the critical 

 temperature. He showed that this tem- 

 perature is constant, and differs with each 

 substance, and that it is always associated 

 with a definite pressure peculiar to each 

 body. Thus the two constants, critical 

 temperature and pressure, which have been 

 of the greatest importance in subsequent 

 investigations, came to be defined, and a 

 complete experimental proof was given that 

 'the gaseous and liquid states are only 

 distinct stages of the same condition of 



matter and are capable of passing into one 

 another by a process of continuous change. ' 

 In 1873 an essay 'On the Continuity of 

 the Gaseous and Liquid State,^' full of new 

 and suggestive ideas, was published by van 

 der Waals, who, recognizing the value of 

 Clausius' new conception of the virial in 

 dynamics, for a long-continued series of 

 motions, either oscillatory or changing ex- 

 ceedingly slowly with time, applied it to 

 the consideration of the molecular move- 

 ments of the particles of the gaseous 

 substance, and after much refined investi- 

 gation, and the fullest experimental calcu- 

 lation available at the time, devised his 

 well-known Equation of Continuity. Its 

 paramount merit is that it is based entirely 

 on a mechanical foundation, and is in no 

 sense empiric ; we may therefore look upon 

 it as having a secure foundation in fact, 

 but as being capable of extension and im- 

 provement. James Thomson, realizing 

 that the straight-line breach of continuous 

 curvature in the Andrews isothermals was 

 untenable to the physical mind, pro- 

 pounded his emendation of the Andrews 

 curves— namely, that they were continuous 

 and of S form. We also owe to James 

 Thomson the conception and execution of a 

 three-dimensional model of Andrews' re- 

 sults, which has been of the greatest service 

 in exhibiting the three variables by means 

 of a specific surface aftei'wards greatly 

 extended and developed by Professor Wil- 

 lard Gibbs. The suggestive work of James 

 Thomson undoubtedly was a valuable aid 

 to van der Waals, for as soon as he reached 

 the point where his equation had to show 

 the continuity of the two states this was 

 the first difficulty he had to encounter, and 

 he succeeded in giving the explanation. He 

 also gave a satisfactory reason for the ex- 

 istence of a minimum value of the product 

 of volume and pressure in the Regnault 

 isothermals. His isothermals, with James 

 Thomson's completion of them, were now 



