14 ELEMENTS OF LABORATORY WORK 



1 cubic decimetre is called a litre. 



1 litre contains 1,000 cubic centimetres or 1,000,000 cubic 

 millimetres. 



1 litre contains 1'76 pints nearly. 



12. Method of Measuring very small Quantities of Matter. 



Although a carefully constructed balance will readily indicate 

 a difference of 1 milligram (or '001 gram) between the quan- 

 tities in the two pans, and although the milligram * weight ' is 

 the smallest quantity of matter which is placed in the pan for 

 the purpose of comparing quantities of matter, yet we have to 

 deal, in chemical measurements, with quantities far more 

 minute than milligrams. In order to obtain this great ac- 

 curacy of comparison, the operation of weighing is combined 

 with that of solution. When a solid is dissolved in a liquid 

 for example, salt in water there are numerous proofs that the 

 solid is evenly distributed through the liquid. A small quantity 

 of the solid is taken and dissolved in a large quantity of the 

 liquid. For example, *1 gram of common salt may be dissolved 

 in some water in a litre flask, and then more water added until 

 it is full up to the litre mark. A portion of this may be trans- 

 ferred to a burette graduated into cubic centimetres, or even into 

 fifths of cubic centimetres. We may thus be certain how much 

 of the salt is contained in our carefully measured portions of this 

 solution j and we may be certain, by using the more accurately 

 graduated burette, to within the 5 o^ro- ( or "00002) of a gram. 



13. Methods of Measuring very small Distances. The 

 Vernier. Greater accuracy in the measurement of distances 

 between two points is obtained when the ordinary scale is 

 accompanied by a sliding scale, or vernier, which is divided 

 so that n divisions correspond with n l divisions of the scale. 



Then each vernier division is -th smaller than a scale division. 



n 



If the vernier has 10 divisions, for example, which are equal 

 to 9 of the scale, a vernier division is y L smaller than a scale 

 division. In use, the vernier is moved until the marked point, 

 or end, is in the proper position for calculating the required 

 dimension. In the diagram (fig. 2) the distance to be measured 



