118 The Plant World. 



outer surface and the wax prevented the solution and migration of 

 the soluble material in the paint. Of course the paint must not 

 extend at all beyond the wax. 



Bv far the best substance so far tested for coating the cup 

 is sulphur. The material is melted to its most liquid form (not 

 too hot), the heated cup is plunged into it, and the surplus sul- 

 phur is brushed off with a hair brush. These coatings have a dull 

 greenish-yellow color, perhaps an undesirable feature, but they 

 operate perfectly. There is no evidence of solution. 



Of the ordinarily obtainable and easily applied substances, 

 natural shellac in alcohol is by far the best. 



Micro-organisms in the cups. — Fungi grow excellently upon 

 the instruments in a moist at^nosphere, and there is little doubt 

 that bacteria also occur, especially on the inner surface. To 

 prevent the growth of these forms a number of treatments have 

 been tried, but nothing is as satisfactory as to rinse the cup with 

 weak mercuric chloride solution before installing. It is true 

 that this adds some soluble salt to the cup, but no after-effects 

 on the coefficient have been detected. Dr. Forrest Shreve 

 informs me that a solution of copper sulphate is also suitable. 

 Formalin can not be used on account of the inevitable presence 

 of paraformaldehyde, which clogs the cups. 



Duplication of instruments. — It has been found desirable to 

 operate two or three cups for each station, placing them side 

 by side aVjout 50 cm. apart. Accidents to several instruments 

 are not likely to occur simultaneously and the attainment of an 

 unbroken record is made more probable by this means. 



Renewal of clogged cups. — This may often be accomplished 

 by grinding. The clogging occurs mainly at the outer surface 

 and this semi-glaze may be completely removed by setting 

 the cup in a lathe (by means of a rubber stopper on a metal rod 

 held by the chuck) and holding fairly coarse sandpaper about it 

 as it revolves. The sandpaper must not be used too long, 

 otherwise the glue of the paper and the heat generated will form 

 a new glazed surface. Cups may be scraped with freshly broken 

 glass or merely rubbed with sandpaper, or they may be ground on 

 an emery wheel. A great many cups which would otherwise be 

 discarded can thus be ground, and after re-coating, will operate 

 as well as new ones. 



