478 M. Regnault's Researclies upon the 



simple phaenomena ; generally speaking, it has to do with com- 

 plex questions, which depend upon many of those laws at one 

 and the same time, and very frequently it is difficult to assign to 

 each its proper influence. The experimenter should therefore 

 endeavour to modify the circumstances in which he operates in 

 such a manner as to vary as much as possible in the several ex- 

 periments the influence of each of the elementary phsenomena 

 and the law which expresses them; He will thus obtain equa- 

 tions of condition, which may be of very great assistance towards 

 discovering the general theory, for tnat, whatever it may be, 

 should always be fulfilled. 



It is from this point of view that I have prosecuted my re- 

 searches ; and I have always striven to define, in the most pre- 

 cise manner, the conditions under which I have operated, in order 

 that whatever theory may finally prevail, some advantage may be 

 derived from my experiments. 



In 1847, I published the first part of my researches; they 

 constitute the twenty-first volume of the Memoir es de VAca- 

 demie. Since that time I have not ceased to carry them on ; but 

 the necessary experiments were so numerous, the numerical cal- 

 culations so long and laborious, that it would have been impos- 

 sible for me to execute them if left to my individual efforts. I 

 have been very considerably aided by M. Izarn, who had already 

 taken part in the first part of my labours, and by a young 

 mining engineer, M. Descos, whom the Minister of Public Works 

 has given me as an assistant, in order to hasten the completion 

 of the investigation. 



The subjects to which my recent experiments relate are the 

 following : — 



1. The relations which exist between the temperatures and 

 tension of a great number of vapours in a state of maximum 

 density, from the most feeble tension to that of 12 atmospheres. 



2. The tension of the same vapoui-s at their maximum density. 



3. The tension of vapour of maximum density produced by 

 mixed liquids. 



4. The latent heat of these vapours under diff'erent pressures, 

 from the most feeble to that of 8 or 10 atmospheres. 



5. The latent heat of vaporization of the same substances in 

 gases. 



6. The specific heat of permanent gases and vapours under 

 different pressures. 



7. The quantities of heat absorbed or disengaged by the com- 

 pression and the expansion of gases, either when that expansion 

 is effected in a space the capacity of which is augmented, or 

 when it takes place during the passage through a capillary 

 opening in a thin partition or through a long capillary tube. 



