chap, x.] CONTINUITY OF THE CHALK. 503 



charged with dissolved gases than surface-waters ; a considerable 

 elevation of temperature being in all cases necessary for the 

 evolution of any dissolved gas. 



Mode of examining Samples. — The samples of water thus 

 taken were examined with as little delay as possible, with a 

 view to determine : — 



(1) The specific gravity of the water. 



(2) The total quantity of dissolved gases contained in them, 



and the relative proportions of oxygen, nitrogen, and 

 carbonic acid. 



(3) The quantity of oxygen necessary to oxidize the organic 



matter contained in the water ; distinguishing between 



a, the decomposed organic matter, and 



b, the easily decomposable organic matter. 



(1) The specific gravity determinations were made at a tem- 

 perature as near 60° Fahr. as possible, with delicate glass hy- 

 drometers, so graduated that the specific gravity could be read 

 off directly to the fourth decimal place with ease. 



(2) The apparatus for the analysis of the gases dissolved in 

 the sea-water was essentially that described by Prof. Miller 

 in the second volume of his ' Elements of Chemistry.' It was 

 found necessary to make several modifications in it, to adapt it 

 to the motion of the vessel. These consisted chiefly in sus- 

 pending much of it from the cabin-ceiling, instead of supporting 

 it from beneath, and in rendering all the parts less rigid by a 

 free use of caoutchouc tubing, &c, the utmost care being taken 

 to keep all joints tight. 



It was found possible to make correct analyses, even when 

 the vessel was rolling sufficiently to upset chairs and cabin- 

 furniture. 



The method of analysis may be thus summarized: — From 

 700 to 800 cubic centimetres of the sample to be examined 

 were boiled for about thirty minutes, in such a way that the 

 steam and mixed gases evolved were collected over mercury in 

 a small graduated Bunsen's gas-holder, all access of air being 

 carefully guarded against. The mixed gases were then trans- 

 ferred to two graduated tubes in a mercurial trough, where the 



