Makoh 23, 1900.] 



SCm'NGE. 



447 



ical aud chemical aspects. Purification thus 

 appears a process that can only be worked 

 by degrees, so that we can consider the 

 absolutely pure body wholly as a limiting 

 condition. In the natural sciences it is 

 often desirable to obtain as closely as possi- 

 ble such limiting conditions : for example 

 we may mention the absolute zero, the pos- 

 sibility of which is clearly deducible by a 

 simple consideration of the law of Gay Lue- 

 sac : the absolute vacuum, a conception 

 usually derived from the fact that the quan- 

 tity of substance in a given space can be 

 more and more diminished ; (we are on the 

 other hand convinced with certainty that 

 it is as little possible to obtain a true 

 vacuum artificially as it is to conceive of it 

 in interplanetary space). Again, the con- 

 dition of absolute equality of temperature 

 between two bodies in contact, or of a sys- 

 tem which is in absolutely complete chem- 

 ical equilibrium, are purely ideal limiting 

 conditions, which, practically speaking can- 

 not be attained. In this sense it is not 

 possible for us to obtain any substance com- 

 pletely free from impurities, and we must 

 therefore be contented to carry the purifi- 

 cation as far as possible — ^a task with which 

 chemist and physicist are employed. A 

 few striking examples may be noted in this 

 field, and especially those in connection 

 with the name of Stas. At first for the 

 purpose of carrying out his atomic weight 

 determinations there was demanded ex- 

 ceptionally pure substances, and it was 

 naturally necessary for him to devise the 

 means of obtaining them. Later he took 

 up the experimental proof of the problem 

 which had been suggested by Lockyer as to 

 the composite nature of certain metals, and 

 for this purpose Stas prepared with the 

 greatest care a series of pure salts. For 

 example, after eleven years' labor he suc- 

 ceeded in preparing a specimen of potassium 

 chlorid, in which it was not possible to de- 

 tect by any means the slightest trace of 



sodium. The investigations of Kohlrauch 

 and Heydweiller may be mentioned here. 

 They undertook the preparation of pure 

 water in order to be able to determine its 

 electrical conductivity with the greatest 

 possible exactness. These investigators 

 were obliged to carrj' out their work in 

 glass vessels, and under such circumstances 

 we can but be astounded that they succeeded 

 in obtaining water so pure that a liter con- 

 tained the exceedingly small quantity of a 

 few thousandths of a milligram of foreign 

 substance. More recently W. Spring has 

 been engaged in this same problem of the 

 purification of water, and has thrown new 

 light upon it. He has found that in all 

 liquids which have been purified by distilla- 

 tion (that is, according to the methods 

 commonly in use up to this time) small 

 particles are still present which may be de- 

 tected by optical methods, and which in 

 some circumstances can be eliminated by 

 an electrolytic pi'ocess. 



Thus these new and more accurate 

 methods furnish results which now in one 

 direction and now in another can be con- 

 sidered more exact, although the attain- 

 ment of the limit, as already mentioned, 

 must be regarded as impossible. In other 

 words, we are not in a position to abso- 

 lutely exclude from a given space which is 

 filled with one substance a large number of 

 other substances. Nevertheless perhaps, 

 in a single instance it has been possible. 

 To touch upon this briefly, we must notice 

 the problem which has been handled with 

 remarkable accuracy by Baker, an English- 

 man. Baker has set for himself primarily 

 the problem of the relations of gases to each 

 other in conditions of the greatest possible 

 freedom from moisture. In passing it may 

 be stated that these gases were kept in 

 fused glass tubes for months in contact 

 with the best drying substances, for example 

 phosphorus pentoxid, and thereby acquired 

 in most cases very remarkable properties. 



