BETWEEN THE CONDITIONS OF A CHEMICAL CHANGE AND ITS AMOUNT. 205 



their course. That which was finally adopted for this and the concluding set may be 

 here briefly described. Different series of experiments difi"ered from one another only in 

 the proportion of sulphuric acid employed, and the several experiments of the same 

 series differed only in the time during which the reaction was allowed to proceed. One 

 description therefore will serve for all ; and to render this description more precise, let 

 it refer particularly to the eighth series of the present set. In the first place, a mixture 

 was made to serve for all the experiments of the series by bringing together in a large 

 flask 200 cub. centims. of the standard solutions of oxalic acid and manganous sulphate, 

 each of these solutions containing in 10 cub. centims. the amount required for one expe- 

 riment, 400 cub. centims. of standard sulphuric acid containing in 10 cub. centims. four 

 units, and 3000 cub. centims. of water. This mixture was exactly divided by means of 

 a narrow-necked flask marked to hold 190 cub. centims., among twenty flasks each of 

 about 300 cub. centims. capacity. One of these was taken and its contents brought 

 to a temperature of 16° C. It Avas then placed in an arrangement for keeping the 

 temperature constant, consisting of an inner and outer casing, supported over a lamp, 

 the interval between which was filled with water at a temperature of 16°. The inte- 

 rior of the inner casing was lined with cotton-wool, and it was provided with a lid 

 similarly lined, which could be slipt over the neck of the flask. A delicate thermo- 

 meter passed into the fluid through a disk of cardboard, by which the flask's mouth 

 was closed. The standard solution of potassic permanganate was contained in a bottle 

 encased in the same manner, and was maintained at the same temperature. A clock 

 beating seconds served to measure the duration of the experiments. When the ther- 

 mometer stood constantly at 16°, a 10 cub. centims. pipette was charged with the 

 permanganate solution, the flask was removed for a moment from its receptacle, and the 

 liquid in the pipette allowed to run into it, starting by beat of the clock at the beginning 

 of a minute. By shaking round the flask as the liquid entered, the whole was perfectly 

 mixed at the end of about ten seconds. If the temperature had suffered any disturbance, 

 it was raised or lowered by the application of the hand or of a damp cloth. The flask 

 was then replaced, and required but little attention until the time for stopping the 

 reaction approached. 



For determining the residue, we have made use of a standard solution of sodic hypo- 

 sulphite. This reagent, according to our experience, is liable to a gradual alteration by 

 which its reducing power is slightly diminished, a few particles of sulphur being at the 

 same time separated from it. The addition of a minute quantity of caustic soda 

 prevents the occurrence of this change, and renders the value of the solution absolutely 

 constant. Just before the allotted time had elapsed a measure of potassic iodide solu- 

 tion, containing rather more than enough to reduce the permanganate originally taken, 

 was poured into a test-tube ; the flask was then removed from its casing, and at the proper 

 moment the contents of the test-tube were discharged into it. By a single rapid shake 

 the iodide is diffused through the liquid, and all the molecules of peroxide which have 



