542 Intelligence and Miscellaneous Articles. 



no printers' mistakes which are manifest upon an ordinary exami- 

 nation, possibly a working student may be able to detect a few 

 errata. Should a new edition be called for, it would not now be a 

 difficult matter to draw up an index. It gives one an idea of the 

 detail with which mathematical subjects are now treated when an 

 index to a text-book is suggested as an almost indispensable 

 adjunct. 



LX. Intelligence and Miscellaneous Articles, 



ON THE ELECTRICAL CONDUCTIVITY OF WATER DISTILLED 

 IN VACUO. BY F. KOHLRAUSCH. 

 ^HE question of the electrical conductivity of water is not yet 

 •*- settled. By a number of precautions, and by particular care in 

 the preparation and conservation of water, I succeeded in obtain- 

 ing it with a conductivity at 22° of only 72 billionth s of that of 

 mercury*. But even if this, or an adjacent value, was repeatedly 

 the lowest attainable limit in various distillations, it could not be 

 maintained that it really represents the conductivity of water, for 

 the preparation of perfectly pure water in the ordinary way seems 

 bound up with insuperable difficulties. 



One of the difficulties consists probably in the influence of air 

 in the distillation. Just as in rain-water compounds of nitrogen, 

 hydrogen, and oxygen, for instance nitrite of ammonia, have been 

 found, so also- even in artificial distillation traces of similar 

 products might occur. 



It is also possible that the mere absorption of air produces the 

 conductivity or, at all events, increases it. On the same occasion 

 I showed that electrolytes got their conductivity mainly by ad- 

 mixture, and thus the addition of a gas, even if it itself does not 

 conduct, might exert an influence. The investigation of this 

 question with the air-pump failed at that time in consequence of 

 other errors thereby introduced. 



It is found, in fact, that the distillation of water in vacuum 

 leads to a far smaller conductivity than that previously found. 



A distilling apparatus in the nature of a water, however, is 

 comparatively easy to construct. A glass vessel of 100 to 200 c. c. 

 capacity, which is to serve as retort, is connected by a glass tube 

 with a smaller vessel, which is provided with two platinized 

 electrodes, each of about 5 square centimetres surface. The 

 resistance of the latter vessel, when filled to various heights, was 

 ascertained by means of an extremely dilute solution of sal am- 

 moniac, the resistance of which was known independently. The 

 vessels were then carefully washed out. 



By a still open tube the two connected vessels were supplied 

 with a suitable quantity of what was already very pure water ; the 

 tube was connected with the mercurial pump, and the water allowed 

 to boil in its vacuum, and when the boiling ceased it was allowed 

 to evaporate at a moderate temperature for about a quarter of 

 an hour, with frequent shaking. Cooled sulphuric acid absorbed 

 * Pogg. Ann. vol. clix. p. 270. 



