164 Mr. F. F. Blackman. Experimental Researches [Dec. 6, 



possible consistent with the well-being of the part, in order that 

 changes in the composition of the gas shall, as soon as possible, be 

 felt by the current which passes thence through narrow tubes to the 

 absorption chambers. When titrations are being made, and the air cur- 

 rent can no longer be allowed to pass through the absorption chambers, 

 it passes through a column of water equal in its resistance to that of 

 the baryta solution in the absorption chamber. This enables the 

 rate of flow to be kept constant between, as well as during, the actual 

 experiments. Numerous other details, such as the special method of 

 refilling the burettes, &c., and above all those small points by which 

 constancy is, as far as possible, attained, many of which have involved 

 weeks of special experiment, cannot be described here. 



Simplification of technique by complication of apparatus has been 

 the guiding principle, and the result is that, although the whole con- 

 sists of at least eight separate pieces of apparatus, many being 

 further in duplicate, and all connected together by a plexus of tubes, 

 yet the working is so automatically arranged that the operator, 

 beyond reading the burettes and occasionally working a finger bellows, 

 has nothing to do but turn stopcocks. 



If only one series of estimations is being made, these can be kept 

 absolutely consecutive, the current being led through one of the 

 absorption chambers, while the solution in the other one is being 

 titrated and renewed, and so on alternately. When two series of 

 comparative estimations are being made at once, a small interval 

 must be allowed after each double estimation, during which the 

 titrations are performed ; the currents of air in connection with the 

 plants then pass through bye-paths, still at their previous rate. This 

 interval (in which a double titration, emptying and refilling of the 

 absorption chambers, is accomplished) can be reduced to ten minutes. 



Delicacy of estimation sufficient for present work is obtained by 

 the use of half-decinormal, N/20, standard solutions. Phenolphtha- 

 lein is used as indicator, and specially delicate end-reactions can be 

 obtained, since atmospheric C0 2 is excluded, and moreover the 

 burettes containing both the solutions can be drawn upon. 



The burettes, narrow and graduated in 1/10 c.c., are read to 

 1/100 c.c., with a simple arrangement for avoiding parallax. All 

 other usual precautions are taken, and series of control titrations, 

 with an error of observation of not more than O'l per cent., are often 

 obtained. This corresponds to 1/200 c.c. C0 2 . 



In experiments of short duration, 1/50 c.c. C0 2 is found to be suffi- 

 cient for a trustwoithy estimation from which definite conclusions may 

 be drawn. 



The communication immediately following the present one, illus- 

 trates the applicability of this apparatus to the investigation of minute 

 quantities of carbon dioxide. 



