April 11, 1913] 



SCIENCE 



561 



found to have the same charge in the two 

 cases. 



In connection with Professor Kimball's re- 

 marks it is of interest to consider also the 

 behavior of two electrometers E and E', one 

 of them, E, fixed within the condenser with its 

 terminals rigidly connected to the armatures 

 A and B, and the other, E', fixed to the mag- 

 net, with terminals sliding on A and B. In 

 Case II. E' indicates a voltage equal to the 

 motional electromotive force in the wire 0, 

 and E gives no deflection — not because the 

 condenser is uncharged, hut hecause the mo- 

 tional electromotive force in the electrometer 

 and its leads just balances the voltage in the 

 electric field produced between A and B by 

 the motional electromotive force in the wire C. 

 In Case I. E again gives no deflection, there 

 being now no electric field between the arma- 

 tures and no charge at all on the armatures; 

 and E" gives the same deflection as before — - 

 but whether for the same reason or not is still 

 an open question. It is apparently because 

 Professor Kimball was considering these am- 

 biguous electrometer indications instead of the 

 actual charges on the cylinders that he was 

 led to his conclusion. S. J. Barnett 



The Ohio State TJniveksity, 

 March 18, 1913 



A labeling surface for laboratory 



GLASSWARE 



The ground-glass circular spot now gen- 

 erally furnished on flasks and beakers made 

 of Jena glass suggested to the writer the 

 desirability of a similar labeling surface for 

 microscopic slides, test tubes and other labora- 

 tory glassware. Attempts to use hydrofluoric 

 acid showed that the acid in solution would 

 dissolve the glass but would not etch it. 

 " Diamond Ink " made by Merck and obtained 

 from Eimer and Amend was found to give 

 satisfactory results. " White Acid," obtained 

 from the same firm, produces a less heavily 

 frosted surface, but has been used to dilute 

 the diamond ink when the latter has become 

 unduly^ thickened. Diamond ink comes in 

 gutta-percha bottles and etches glass imme- 

 diately upon coming in contact with it. The 



hydrofluoric acid, which is apparently one of 

 the constituents, is volatile. When a bottle 

 has been opened, the fluid tends to creep out 

 by capillarity along the salts that have been 

 deposited by evaporation. The bottles, in con- 

 sequence, should be kept sealed with parafiine 

 when not in use. 



In using, a small amount only of the creamy 

 diamond ink is poured into a stender dish 

 which has been previously coated with paraf- 

 fine. A rubber stopper has been found to be 

 the best means of applying the ink. One end 

 is dipped into the ink and then pressed against 

 the glass to be etched. If the right edge of 

 the stopper is first touched to the glass and 

 the pressure shifted from right to left and 

 reversed, the fluid is evenly distributed and 

 a small amount only is necessary for the even- 

 edged circular spot which results. The etch- 

 ing takes place at once. The surface, how- 

 ever, is covered by a thick deposit of deliques- 

 cent salts which must be washed off before 

 the ground-glass surface is ready to receive 

 pencil marks. Ordinary glassware is easily 

 marked by the method outlined, but the Jena 

 glass tested is etched with more or less difli- 

 culty and has been ground on an emery wheel. 



Adhesive paper labels are impracticable for 

 test tubes or flasks that have to be sterilized 

 with steam or that are kept in a moist atmos- 

 phere where they are liable to the attacks of 

 moulds. Wax pencils, or better, indelible copy- 

 ing pencils moistened with alcohol are useful 

 for temporary labels, but do not withstand 

 steam sterilizing and are not permanent when 

 much handled. Marking diamonds and cer- 

 tain silicate inks have the disadvantage of 

 leaving a written label that can not be re- 

 moved. A label written with lead pencil on 

 an etched or ground-glass surface, however, 

 has the advantage of permanency so far as 

 ordinary laboratory handling is concerned. 

 It is as permanent as pencil marks on paper 

 and, like these, can be removed with a rubber 

 eraser. The pencil marks are not afl^ected by 

 water nor by steam, but may be readily re- 

 moved by scouring soaps. In cleaning test 

 tubes, it has been found convenient to remove 



