174 ELEMENTS OF LABORATORY WORK 



A filter paper on each pan is advisable for direct weighing. It must 

 be remembered that some substances vary in mass during the opera- 

 tion of weighing for example, hot water rapidly loses matter by 

 evaporation, while calcium chloride would gain by absorbing 

 moisture. 



Finally, great care is needed in supervising the manipulation of 

 weights, and in exacting that they are properly used, and properly 

 returned to their right position in the box after every operation. 

 No excuse should ever be admitted for neglect of this important rule. 

 The weights should be counted when in the pan, and also when they 

 are returned to their places. Before using the weights it is advisable 

 for each student to add together all the weights in the set, giving to 

 the smaller weights their correct decimal values. 



2. Observation of Dimensions and Densities.- Besides ordinary 

 steel or box-wood scales, it is advisable to have brass pieces let into 

 benches where convenient, to denote distances of a metre and a 

 decimetre. Also longer distances, marked along walls, are often 

 convenient for measuring wires. Standard measures should be always 

 at hand, and as conspicuous as possible, in order that students may 

 become familiarised with them. By encouraging guessing before 

 actual comparison, great help is given in training the eyes to measure 

 distance. This should also be carried on in comparing the capacities 

 of vessels, or the volumes of solids. This is a most valuable kind of 

 training, which should always be carried on in a laboratory, side by 

 side with the more systematic work. The exact form of such indirect 

 training will vary with circumstances and with teachers, but the 

 opportunity of making it very powerful and suggestive will always 

 be present. 



Burettes are very convenient for delivering known quantities of 

 liquids ; they should be supported by clamps before windows or white 

 walls if possible. The form with caoutchouc tube and clip for 

 delivery is better for beginners than the one with a tap. The use of 

 a float, for more exact reading, may be encouraged. Besides burettes, 

 a small number of graduated cylinders will be required for measuring 

 the volumes of liquids poured into them. A variety of graduated 

 flasks, including some small enough to be weighed on the balances, 

 will also be wanted. 



3. Observations of Temperature-changes. Thermometers. -The most 

 serviceable thermometers are the narrow chemical thermometers 

 reading from 20 below zero to 200 Centigrade. Occasionally a 

 wider range is required. The absolute value of the readings may 

 be found to be somewhat inaccurate when tested by the standard 



