39 2 MR. J. J. MANLEY: OBSERVATIONS ON THE 



Very similar curves were obtained as the result of experimenting with loads 

 ranging from 10 to 200 gr., and, with only one or two exceptions, the other balances 

 which were tested in this research behaved in a similar manner. 



Of the Conditions under which the Experiments were Conducted. 



In the preliminary experiments the balances were used under the more or less 

 favourable conditions which obtain in a laboratory where quantitative analysis is 

 carried on. But all subsequent and final experiments, from which our conclusions 

 were drawn, were conducted in a room having a N.E. aspect, and possessing, in 

 general, a very steady temperature ; and in order to maintain as constant a tempera- 

 ture as possible within the balance case, a temporary cupboard was, in every instance, 

 built up round the instrument. This cupboard was constructed of sheets of thick 

 asbestos and plate glass, the interspaces being, to a large extent, loosely packed with 

 cotton wool. 



The temperature within was indicated by two sensitive thermometers made of Jena 

 " normal " glass. These thermometers, previously standardized, were placed parallel 

 to the length of the beam, the bulb of the one being brought as close as possible to 

 the end of the left-hand, or L, arm, whilst that of the other was similarly placed with 

 regard to the right-hand, or II, arm. By means of a telescope the temperatures were 

 estimated to 1/100 C. 



A little experience showed that a distinct advantage was gained by illuminating 

 the room with yellow light only. 



The monochromatic light was conveniently obtained by making use of a 6 -litre 

 flask, which was nearly filled with a solution of potassium chromate of suitable 

 strength. A wide glass tube, closed at the lower end, was introduced into the flask, 

 and inside this was placed a glow-lamp of cylindrical form. The flask was then 

 supported in a position immediately opposite the centre of the balance case, and at a 

 distance of about one metre. This method of illuminating the scale proved highly 

 satisfactory, chiefly on account of the ease witli which a sharp image of the pointer 

 could be obtained in the reading telescope. As the heat rays emitted by the lamp 

 were absorbed by the chromate solution, the balance was not affected by those 

 disturbing influences which result from the near presence of a lamp under ordinary 

 conditions. Finally, the screening of the window to exclude extraneous light helped 

 very materially to maintain for short periods the desired uniform and constant 

 temperature. 



Some Experiments ivith a Circular .Beam. 



One of the first instruments experimented with under the conditions just described 

 was a balance having a circular beam. Numerous tests with loads varying from that 

 of the pans alone (commonly termed zero load) to the maximum of 200 gr. were 



