PRINCIPLES OTF HEATING HOT-HOUSES. 161 



was ascertained by Petit and Dulong, in a series of experiments, 

 not necessary here to detail, but which proved the accuracy of 

 the deduction. 



By comparing the second and third columns of the above 

 table, it will be immediately perceived that the loss of heat by 

 radiation varies greatly, with the nature of the radiating sur- 

 face ; though, whatever be the nature of the surface, the loss of 

 heat is the same in all cases, though in a different ratio. 



It should be observed, that, in this table, the second, third, 

 and fourth columns show the number of degrees of heat which 

 were lost per minute by the body which was subject to the 

 experiment ; and, therefore, these numbers represent the velocity 

 of cooling. 



The fact, already adverted to, that the ratio of cooling in 

 those bodies that radiate least is more rapid at low tempera- 

 tures, and less at high temperatures, than those bodies that 

 radiate most, is, perhaps, one of the most remarkable of the laws 

 of cooling. It was first deduced experimentally by Petit and 

 Dulong, arid it may be mathematically proved from their for- 

 mula ; but it is unnecessary here to enter into the investigation. 

 It appears, however, that when the total cooling of two bodies is 

 compared, the law is more rapid at low temperatures for the 

 body which radiates least, and less rapid for the same body at 

 high temperatures ; though separately, for conduction and radia- 

 tion, the law of cooling is, for the former, irrespective of the 

 nature of the body, and for the latter, that all bodies preserve at 

 every difference of temperature a constant ratio in their radi- 

 ating power. 



It is not our purpose to enter minutely into detail on the laws 

 of heat, which will be found in modern works on chemistry, 

 and which ought to form part of the studies of all young gar- 

 deners who wish to become acquainted with the principles of 

 hot-house management. We will now proceed to consider the 

 specific properties of air and water as agents in the heating of 

 horticultural structures. 



3. Specific heat of air and water. Very erroneous notions 

 are entertained by many persons as to the absolute quantity of 



