156 MR. T. E. THORPE ON THE AMOUNT OF CARBONIC ACID 



tions SO minute as the absorption of the carbonic acid 

 from a small volume of atmospheric air^ and when we re- 

 member the fact pointed out by Regnault in the investiga- 

 tion above mentioned (Ann. de Chimie et de Phys. xxxvi. 

 1 85 2) J that air which has remained for any great length of 

 time in glass tubes invariably exhibits a notable diminution 

 in the amount of carbonic acid, since the glass absorbs a 

 portion of this gas. 



The kind permission of the Honourable Board of Trinity 

 House has enabled me, during the vacation of last summer, 

 to make some additional experiments in this direction, on 

 board the Bahama-bank light-vessel, situated in the Irish 

 Sea,lat. 54° 21', and long. 4° 11', seven miles W.N.W. of 

 Ramsey, Isle of Man, and consequently nearly equidistant 

 from the nearest shores of England, Scotland, and Ireland. 

 The ship is placed to mark the proximity of a dangerous bank, 

 by which, for the greater part of the day, a strong current, 

 setting in from the southward, flows through the north 

 channel, and thence into the Atlantic. 



These experiments were made in the early part of August, 

 at the same periods of the twenty-four hours namely, about 

 4 A.M. and 4 p.m., or nearly the times of minimum and 

 maximum temperature. 



Pettenkofer's method of analysis was adopted, with the 

 improvements in the practical details suggested by Angus 

 Smith (Proc. Lit. and Phil. Soc. Manchester, 1865). This 

 method is in principle similar to the one employed by 

 Watson and Emmet, but admits of far more delicacy and 

 precision in practice. Baryta- is substituted for lime-water, 

 and oxalic for sulphuric acid. The solution of oxalic acid 

 for these experiments was so made that one cubic centi- 

 metre of it corresponded to one milligramme of carbonic 

 acid ; it thus contained 2*864 grammes of pure crystallized 

 oxalic acid per litre. Twenty-five cubic centimetres of the 

 baryta solution were originally made to correspond to 



