DILUTION OF CONCENTKATED SOLUTIONS. 323 



spray, and the water is projected out. (Diagram I., P.) The process of mixing is 

 then carried out, and after a sufficient time the temperature change is noted. Some 

 estimate is made of the water in the pipette which has not been driven out, viz., that 

 wetting the walls of the tube. For this purpose, the tube being removed, dried 

 externally and weighed, allowance is now made in the amount of water quoted. 



Thermal Capacity of Vessel and its Contents. 



The thermal capacity of the vessel and its contents must be very carefully 

 determined, a process which is always difficult. When a glass mixing vessel is 

 employed special care has to be taken in the determination, as glass lias a high specific 

 heat. Moreover, its low conducting power involves the temperature not being so 

 nearly equal throughout its mass. Thus it was found that the thermal capacity 

 differed with the amount of solution taken. In all cases quantities of solution taken 

 were of either 250, 275 or 300 c.c., the amounts being measured by standardised 

 measuring vessels. 



Thermal capacities were found for the vessel and its contents also, when the mixing- 

 pipettes were present or absent. To ensure accuracy experiments were repeated. 

 Consistent values were obtained. "Roughly speaking the thermal capacity was of the 

 order of one-tenth that of the total thermal capacity. 



Mode of Heating. 



The method of determining the thermal capacity of the vessel under the varying 

 conditions, was that of heating by a known electric current a weighed quantity of 

 water contained in the vessel. Some difficulty was experienced in the construction of 

 a satisfactory heating coil. The difficulty was solved by selecting a piece of glass 

 tubing of very thin walls, and elliptical in section. (Diagram I., H.) Manganin 

 wire was wound upon a flat strip of mica and inserted into the tube, one end of which 

 was sealed. Good thermal contact between the coil and the walls of this tube was 

 ensured by filling up the tube with the best lubricating oil, and the upper end was 

 lightly close with glass wool. The coil so made was thus perfectly insulated, and yet 

 presented a large heating surface. There was found to be very little lag in the 

 taking up of the heat by the solution. The length of the coil was so taken as to 

 extend throughout the depth of the solution. 



The heat generated was measured by the products . where C is the current, E 



4 J. o 



the volts, t the time in seconds, and 4'185 the mean value of JOULE'S equivalent. A 

 Weston ammeter and a Weston voltmeter were employed having ranges from to 5 

 in both cases. It was found possible to read to O'OOS ampere or volt with fair 

 accuracy. Both instruments were calibrated against standards and the observations 

 quoted are those observed, to which the requisite correction is applied. The 



