ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
697 
admirable simplicity for insuring this result. But I have found that in 
practice the use of the differentiator involves a considerable expenditure 
of time. To get a specimen from distilled water to 90 per cent, 
alcohol for example, no fewer than eleven different mixtures of water 
and alcohol have to be made up and poured into the reservoir tube. 
A simple piece of apparatus which I have devised does away en- 
tirely with this— the gradual substitution for one another of the two 
fluids of different densities being effected automatically. An obvious 
mode of meeting the difficulty suggests itself at once. Why not have 
the second fluid falling into the first drop by drop, mixing thus very 
gradually with it and eventually replacing it ? The difficulty in the 
way of this is that as each drop of the much lighter liquid enters the 
denser, violent though circumscribed currents are produced which are 
damaging to the delicate organisms we are dealing with. 
The requisites for the method about to be described are — several 
reservoirs of glass or earthenware fitted with glass taps and having 
each a capacity of a gallon or more, 
some wide-mouthed bottles of a va- 
riety of sizes, fitted with perforated 
india-rubber stoppers, and some 
lengths of glass and india-rubber 
tubing. 
Two bottles of similar size are 
connected together by tubing in the 
way represented in the woodcut. 
One of these A we call the mixing 
bottle ; the other B contains the 
objects, and must have a capacity 
(fig. 83) equal to at least a hundred 
times the bulk of the latter. The 
objects are in fluid 1, and it is de- 
sired to substitute fluid 2. Both 
bottles are filled, or partially filled, 
according to circumstances, with 
fluid 1, and bottle A is connected 
with a reservoir of fluid 2. It is 
somewhat difficult by means of a tap 
to regulate the flow so that, let us 
say, one drop in five seconds will pass 
out of the reservoir ; and it is much 
more convenient to effect this by 
intercalating in the supply pipe a 
section of glass tubing drawn out 
to the required degree of fineness 
(represented in the figure as discon- 
nected from the proximal portion 
of the supply tube). The rate of 
flow through this narrow section of 
the tube can be further regulated by raising or lowering the reservoir 
or the mixing bottle, thus altering the pressure. With bottle B is con- 
nected an overflow tube. Above the narrow section of glass tubing in 
Fig. 83. 
