1865.] Physics. 129 



platinum maybe thus expressed : — dividing the radiation into 24 equal 

 parts, one of these parts is luminous and 23 obscure. When instead 

 of the white-hot platinum the most brilliant portion of the flame of 

 coal gas is employed, and the radiation is divided into 25 equal parts, 

 one of these is found to be luminous and 24 obscure, whilst if the 

 radiation from an electrical light excited by 4 J cells is supposed to 

 be divided into 10 equal parts, one of these parts is luminous and 9 

 obscure. 



In a solution of iodine the experimentalist has now the means of 

 almost perfectly detaching the obscure from the luminous heat rays of 

 any source, and Dr. Tyndall has described numerous experiments 

 in which brown paper was set on fire, gun-cotton exploded, and plates 

 of tin and zinc fused, by placing these substances at a certain point 

 in the air of a perfectly dark room, where the invisible rays of the 

 electric light were concentrated. Dr. Tyndall has recounted one bold 

 experiment on the amount of heat rays which the human eye could 

 bear without injury. Converging the beam from the electric lamp by 

 a glass lens, the opaque solution of iodine was placed before his 

 open eye, and the eye was brought into the focus of the obscure rays, 

 the heat was immediately unbearable ; but it seemed that the un- 

 pleasant effect was mainly due to the action of the obscure rays upon 

 the eyelids and other opaque parts round the eye. He therefore cut 

 in a card an aperture, somewhat larger than the pupil, and allowed 

 the concentrated calorific beam to enter the eye through this aperture ; 

 the sense of heat entirely disappeared. Not only were the rays thus 

 received upon the retina incompetent to excite vision, but the optic 

 nerves seemed unconscious of their existence even as heat. What the 

 consequences would have been had Dr. Tyndall permitted the luminous 

 third of the condensed beam to enter the eye, he is not prepared to 

 say. 



Mr. Barrett, assistant in the Physical Laboratory of the Royal 

 Institution, has applied some of Dr. Tyndall's researches to the analy- 

 sis of the human breath, and with very remarkable results. The mode 

 of analysis is founded upon the calorific absorption exerted by the 

 carbonic acid contained in the breath. With ordinary sources of heat, 

 carbonic acid is probably the most feeble absorbent amongst the com- 

 pound gases, but Dr. Tyndall has shown that where a carbonic oxide 

 flame is used as the source of heat, carbonic acid instantly reverses its 

 position, and exceeds all other gases in its heat-absorbing power. By 

 employing a small carbonic oxide flame as the source of heat, and pass- 

 ing pure dry air into the experimental apparatus, no absorption what- 

 ever of heat was noticed, but common air gave an absorption of 15 per 

 cent, of the total radiation ; air, deprived of its aqueous vapour, but 

 retaining its carbonic acid, gave 13-8 per cent, absorption ; whilst air 

 deprived of its carbonic acid, but retaining its aqueous vapour, gave 

 only 6*4 per cent. By this method very small variations in the amount 

 of carbonic acid present can be detected, and by working with air con- 

 taining known quantities of carbonic acid, it was easy to construct a 

 scale by comparison with which the percentage of carbonic acid in 

 human breath could be' ascertained with great accuracy. A series of 

 such analyses are given, with the corresponding amounts of carbonic 

 VOL. II. k 



