Notes. 
163 
flows continuously, and through which the air is drawn. The C0 2 
generator is constructed on a new principle, and consists of a tall 
tube containing fragments of marble, through which the air current 
passes at a constant rate, while very dilute HC1 trickles down it at an 
extremely slow rate, which is made constant and independent of 
external variations of temperature by special arrangements. Thus a 
constant amount of C0 2 is being continually generated, and is carried 
off by the air current. The amount of C0 2 formed can be controlled 
by the strength of the acid employed. When generating amounts 
below 2 per cent, of the air current this arrangement works very 
constantly. From the C0 2 generator or remover, as the case may be, 
the current of air passes to the receivers, in which the parts of the 
plant under investigation are situated. These receivers are of various 
forms, according to the material experimented on, but are all con- 
structed on the cardinal principle of making them as small as 
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 
M 2 
