1880.] Measurement of Actinism of Suns Rays and Daylight. 355 



was removed from the jar, baryta water was introduced and well 

 shaken, and after standing a sufficient time to effect the absorption of 

 the carbonic acid was titrated in the usual manner. 



The results obtained were highly interesting, but it would not 

 at present be prudent to speak of them except in general terms. It 

 will suffice to say that the experiments showed that if the plant did 

 not actually absorb carbonic acid during the day, it exhaled none ; 

 while at night large quantities were so got rid of — thus fully sub- 

 stantiating the generally accepted view of the matter in point. 



As a mean of seven night experiments made between 23rd August 

 and 29th September, there were 12'18 vols. C0 2 in 10,000 of air 

 found in the jar. The largest quantity present was 14"9 vols., and the 

 least 9*13 vols, in 10,000 vols, of air. The former was at an early 

 period of the experiments, the latter towards their close when the 

 plant had lost a few of its leaves and was beginning to show a dimi- 

 nished activity generally. 



Summarising the results contained in this communication, it may be 

 stated — 



(1.) That the normal amount of carbonic acid present in the air of 

 the land is distinctly less than that usually stated, and that it does 

 not exceed 3*5 vols, in 10,000 of air. 



(2.) That plants absorb carbonic acid during the day and exhale it at 

 night, and that vegetation, therefore, affects the quantity of carbonic 

 acid present in the air, decreasing it by day and increasing it at 

 night. 



(3.) That from this cause there is, during that part of the year 

 when vegetation is active, at least 10 per cent, more carbonic acid 

 present in the air of the open country at night than during the day. 



III. " Measurement of the Actinism of the Sun's Kays and of 

 Daylight," By Dr. R. Angus Smith, F.R.S.' 



(Preliminary Notice.) 



When examining the air of towns and the effect of smoke and fogs, 

 I have often wished for a very simple chemical method of measuring 

 the total light absorbed by these gases, vapours, and floating solids. I 

 do not undervalue the work of others, but I think I have obtained a 

 process promising good results with great simplicity, although I dare 

 say it introduces its own class of difficulties. 



1. The fundamental fact is that when iodide of potassium in solution 

 is treated with nitric acid, so small in quantity as to cause no change 

 of colour in dull diffused light, a change takes place when the same 



