62 



NATURE 



\_May 20, 1880 



time Despretz showed that notable variations took place 

 in the case of air above a pressure of fifteen atmospheres. 

 Arago and Uulong, intrusted by the French Academy 

 with the verification of these observations, carried out a 

 carefully conducted series of experiments on the compres- 

 sibilityof air extending up to twenty-seven atmospheres,but 

 came, however, to the conclusion that Mariotte's law was 

 correct. This opinion was strengthened by Pouillet's 

 researches in so far as it related to the then so-called 

 pennanent gases, while confirmatory evidence was brought 

 in favour of Oersted and Schwendsen's experiments on 

 easily liquefiable gases. This view of the correctness of 

 the law for a certain group of gases was held by the 

 scientific world until 1845, when Regnault, by a brilliant 

 series of experiments of the most exact kind, showed that 



air, nitrogen, and carbonic acid experienced a constant 

 decrease of elasticity when submitted to pressures rising 

 to thirty atmospheres, while under the same conditions a 

 regular increase of elasticity in the case of hydrogen 

 occurred. A few years later Natterer of Vienna published 

 some remarkable experiments on the compressibility of 

 gases, making use for the first time of enormous pressures, 

 reaching in several cases nearly 2,800 atmospheres. While 

 Natterer's methods of measurement were by no means 

 exact, the results of his experiments showed beyond 

 doubt that for pressures above eighty atmospheres oxygen, 

 nitrogen, and carbonic oxide possessed the same peculiar 

 property manifested ordinarily by hydrogen, viz., the 

 volume of the compressed gas being greater than that 

 demanded bv Mariotte's law. The verification of Natterer's 



results was undertaken in 1870 by Cailletet, whose name 

 has been so prominent of late years by his success in 

 liquefying the so-called permanent gases. By making 

 use of one of Desgofte's manometers he experimented on 

 air and hydrogen up to 600 atmospheres, and obtained 

 figures comparing very closely with those published by 

 Natterer. 



So much for the data on this subject up to w-ithin a 

 recent period. While the general truth of the results 

 chronicled by Natterer and Cailletet was accepted by the 

 physical world, it was still regarded as of prime import- 

 ance to carry out the experiments under the influence of 

 pressures with regard to the measurements of which there 

 could be absolutely no doubt. The only practicable 

 method of attaining this end was evidently to make use of 

 enormously high columns of mercury. In 1S75 Dr. 

 Andrews attempted the solution of the problem in this 

 manner, but was forced to succumb before the mechani- 



cal difficulties attendant upon its execution. The French 

 I physicist M. E. H. .\magat, who has devoted his atten- 

 tion for a number of years past to the phenomena of com- 

 j pressibility, appears to ha\ e been more successful in 

 \ overcoming the manifold obstacles in the way of accom- 

 plishing the task, and furnishes ' an interesting account 

 of what is certainly one of the most remarlcable (ours de 

 \fo}\'e of modern experimental physics. It consisted in 

 making e.xact measurements of the changes in volume of 

 gases when submitted to the pressure of a column of 

 mercury of over one-fifth of a mile in height. In order 

 to give a correct idea of the conditions under which 

 Amagat's important results have been obtained, we will 

 describe briefiy the three essential elements of the ex- 

 periments : the locality, the column of mercury, and the 

 apparatus for receiving the pressures, communicating 

 them to the gases operated upon, and measuring the 



■ Anii.ihid; ChimL- ,:• d^ rhysljiielil.yX^. 34;. Mars. iSEo. 



