C. GENERAL PHYSICS. 147 



OBSERVATIONS ON THE THEORY OF CROOKES' RADIOMETER. 



Professors Dewar and Tait, in the prosecution of their re- 

 searches into the movements of Crookes' radiometer, have 

 devised methods of producing very perfect exhaustion of air 

 from a receiver; the most perfect vacuum being obtained by 

 taking advantage of the absorbent power of charcoal. The 

 movement of the radiometer, whose disks are of rock-salt, 

 are traced to the unequal heating of the movable parts of 

 the apparatus; and the explanation of the phenomena ob- 

 served by them is deduced from the kinetic theory of gaseous 

 pressure. Proceedings Royal Society of Edinhurgh^ VIII. , 

 628. 



THE MECHANICAL PRODUCTION OF COLD. 



Mr. A. C. Kirk has offered a memoir on the mechanical 

 production of cold to the Royal Institution of Civil Engi- 

 neers, which attracted extended discussion, and will proba- 

 bly lead to material improvements in many of the mechanical 

 arts. He states that his attention was first drawn to the 

 subject by noticing the inconvenience experienced at certain 

 paraffin-oil works, where it was customary to extract the 

 solid paraffin in winter by exposing the material to a low 

 temperature sufficient to cause the paraffin to crystallize. At 

 those works chemical methods of producing low tempera- 

 tures had been introduced in order to avoid the otherwise 

 expensive loss of time. These methods were too objection- 

 able to be continued long, and the author was requested, as 

 engineer to the works, to examine the methods invented by 

 Dr. Gorrie, who had constructed a machine that was said to 

 have produced ice in Florida. Mr. Kirk's early experiments 

 with machines similar to those of Dr. Gorrie havings been 

 unsatisfactory, attention was turned to an air-engine, the 

 reversal of whose processes it was thought ought to make a 

 good cooling-machine; and, in fact, after many modifications 

 and reconstructions, a degree of cold was produced by it 

 sufficient to freeze mercury. This machine may be in the 

 main described as follows : A steam-engine moves a piston 

 backward and forward, by wdiich the confined air is in a 

 state of alternate compression and expansion. While the 

 air is compressed in one space, the heat generated thereby is 



