40 British Association for the Advancement of Science. 



pared, in other respects, before we can estimate the eifects really 

 due to the state of the surface, he must, of course, be understood 

 to speak under the qualification acutely referred to by Prof. Bache, 

 dependent on the fact noticed by Leslie, that radiation takes place 

 not only from the surface, but from a certain minute, though sen- 

 sible depth, which differs m difierent substances. 



Taking this into account, the general meaning, as well as im- 

 portance of the caution, will be manifest. In the sequel, Mr. 

 Bache gives some very precise experimental proofs of the truth 

 of the law just noticed, and shows, by successively adding fresh 

 coats of the pigment, the precise limit beyond which such addi- 

 tion ceases to increase the radiating power, — which, in fact, there 

 comes to a maximum, and with greater thicknesses decreases. 



When this point had been ascertained carefully for each pig- 

 ment, their eifects were observed with great accuracy, and com- 

 pared with a standard surface under similar circumstances ; the 

 observations include a considerable range of substances, differing 

 both in color and other properties. The results exhibit no cor- 

 respondence of the greatness of effect with the color. The source 

 of heat was hot water : — the author allows fully the distinction 

 between properties of heat of this kind, and that connected with 

 light ; in the latter case it is evident that color is an essential ele- 

 ment ; a wide field is yet open for tracing on what the effect does 

 depend : and again, since Melloni has pointed out the existence 

 of many kinds of heat, differing in their relations to screens, to 

 trace also their different relations to surfaces. 



Ui'ic Add and Urea, by Prof Liehig. — The important part 

 which uric acid performs in the animal economy, has for a long 

 time attracted the attention of the most distinguished physicians 

 and chemists. Uric acid forms in one class of animals the whole 

 of the excrement, and in another class it is its principal constitu- 

 ent, and it is accompanied by urea, a never-failing constituent of 

 the human urine. Its extraordinary production in that morbid 

 state of the body, which we call a predisposition to gout, is well 

 known to give origin to one of the most painful diseases to which 

 mankind is liable. It may be affirmed with the utmost certainty, 

 that urea and uric acid are products of the organization. We 

 cannot discover their existence in any part of our food, nor do 

 they constitute a part of any organ, as fibrine does of the blood ; 

 but they are chemical combinations of a peculiar nature, on which 



