BETWEEN THE CONDITIONS OF A CHEMICAL CHANGE AND ITS AMOUNT. 119 
according to circumstances, of a trace of sodic hyposulphite or hydric hypochlorite. If 
at the end of half an hour the blue tint had neither deepened nor disappeared, it was 
considered that the experiment might be proceeded with. Secondly, it was necessary 
to add a little measure of sodic hyposulphite, the first of a series of similar measures 
which were to play the part already indicated in the observation of the course of the 
reaction. These measures needed to fulfil two somewhat incompatible conditions ; they 
must be exactly equal, or at least must stand to one another in a known ratio ; and they 
must be of very small volume in order that their addition might not materially augment 
the total volume of the fluid. In the earlier experiments a pipette of about half a cubic 
centimetre capacity, with a capillary tube at either end, was filled with the hyposulphite 
solution by means of a siphon-tube provided with india-rubber nozzle and clamp. The 
lower end of the pipette having been wiped and pressed gently against a pad of blotting- 
paper, was inserted into the cylinder through the hole in the bung so as to dip beneath 
the surface of the fluid. By applying the mouth to a piece of india-rubber tubing slipt 
over the upper end of the pipette, and alternately blowing and sucking, the charge of 
hyposulphite was driven and completely washed into the great body of the fluid within 
the cylinder. This mode of measurement proved to be susceptible of great accuracy, but 
it only satisfied imperfectly the second condition, that of producing no material increase 
in the total volume of fluid used in the experiment. It will be seen that in some of the 
experiments hereafter recorded as many as twenty observations of the return of the blue 
colour were made ; the total quantity of hyposulphite that had been added at the close 
of such an experiment was therefore 10 cub. centims., causing an increase of 1 per cent, 
in the volume of the fluid. 
Subsequently a method of measurement was devised by which this source of error was 
avoided. It consisted in collecting single drops of a strong solution of hyposulphite 
under circumstances favourable to their perfect uniformity, and introducing these in 
succession into the cylinder. The drops were formed at the end of a siphon of which 
the shorter limb passed into a bottle containing the standard solution, while the longer 
limb, clamped firmly to a solid stand, was protected at its extremity by an outer tube 
which extended slightly below it and served to shield the growing drop from currents of 
air. The siphon was at one point so contracted as to deliver not more than five drops 
in a minute. The drops were received in small tubes about 6 inches long, open at both 
ends ; in the side- of each tube near one extremity a round hole had been blown such as 
Would be made for the purpose of joining on another tube at right angles. Two forks 
were so placed on either side of the long limb of the siphon as that when one of these 
tubes had been passed through and rested upon them it lay horizontally just under the 
dropping-point, and could easily be so adjusted as to receive a drop through its lateral 
opening. When a drop had fallen, the tube containing it was withdrawn and replaced 
by another tube until a sufficient number of drops had been collected. An india-rubber 
cap was then slipped over the dropping-point to stop the flow of the liquid. The whole 
apparatus remained always in readiness, needing only the removal of its cap whenever it 
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