186 Proceednigs of the Royal Society of Edinburgh. 



change in an atmosphere so highly charged with carbonic acid as to 

 prove rapidly fatal to animals. 



It appeared to the authors necessary to ascertain whether this 

 effect took place in pure carbonic acid, containing no free oxygen, as 

 the atmosphere used by Dr Stevens most probably did ; and they 

 also proposed to extend their observations to other gases, likewise 

 free from oxygen. They accordingly prepared pure nitrogen, hy- 

 drogen, and carbonic acid, removing the last traces of oxygen by 

 means of potassium. The water with which the clot was washed, 

 and the saline solution used in the experiments, were deprived of at- 

 mospheric air by being boiled and allowed to cool in close vessels. 

 The clot was then introduced into the gases over mercury, and as 

 soon as the strong saline solution came in contact with it, the colour, 

 in all the three gases, changed from black to bright red, and the 

 same change was found to take place in the Torricellian Vacuum. 

 It was obvious, therefore, that a strong saline solution could change 

 the colour of the blood from venous to arterial without the contact of 

 oxygen, or indeed of any gas whatever. 



But in blood, the colouring matter is in contact, not with a strong 

 saline solution, but with a very dilute one, viz. the serum. It was 

 necessary, therefore, to see whether the washed clot, placed in con- 

 tact with serum or a weak solution of salt, in the same gases, would 

 change its colour. On repeating the experiments, both with serum 

 and a solution of salt in water of equal strength to the serum, no 

 change whatever took place, until atmospheric air or oxygen gas was 

 admitted. 



The conclusions of Dr Stevens, therefore, must be somewhat mo- 

 dified. It is true, as he states, that the presence of saline matter is 

 essential to the change of colour : But it is obvious, that there is an 

 essential difference between that change as it occurs in the lungs, 

 where serum is present, and as it appears out of the body when a 

 strong saline solution is employed. In the former case, oxygen is 

 necessary : In the latter, the change of colour is independent of the 

 presence of any gas whatever. We must, therefore, be cautious how 

 we reason by analogy from the one class of phenomena to the other. 



April 15. — Professor Russell, Vice-President, in the Chair. 

 Xhe following communications were read : — 



1 . Observations on the Lines of the Solar Spectrum, and on 

 those produced by the Earth's Atmosphere, and by the 

 Action of Nitrous Acid Gas. By Sir David Brewster, 

 LL.D., F. R.S. 

 The author was led, in prosecution of his researches on the absorp- 

 tive action of transparent media of light, which have been partly 



