224 



NATURE 



[July 9, 1896 



in England and for the metropolis, only a few minutes before 

 the occurrence of the terrible raihvay accident at Grantham on 

 that night), I would lie glad to know if they confirm the near 

 approach to earth over the Brentwood Hills (as the end-point), 

 and the position near Castor (as nearly the point of first appear- 

 ance), assigned by this discussion to the meteor's flight ; or if, 

 as seen much sideways from its plane of fall, in counties north 

 of Essex, the. meteor's apparent line of flight may have been 

 but little accordant perhaps, or even, quite possibly, not at all 

 conformable with these sky-positions? 



With regard to Mr. Denning's identification of the course of 

 the above described bright meteor of the 13th inst., with a 

 radiant-centre near Honores, I may mention in concluding that 

 on the 19th inst., at 11.56, I noticed here a long-pathed streak 

 leaving, 2nd mag. meteor shoot swiftly (about 30° in l^ sec.) 

 from 195°, -F 68% in Draco, to 181°, -f 39°, in Canes Vciialici, 

 leaving a thin white streak for 2A seconds along its whole track. 

 This course prolonged backwards about 60" proceeds from 

 350°, + 37°, only 3° or 4° from the place at 354°, + 39", of the 

 radiant No. 174 of Mr. Denning's list ; and like the brighter 

 shooting-star of June 13, it evidently belonged to this same June 

 "Ilonorid" system. Mr. Denning's outline of the shower's 

 duration, through the latter half of June and the first week of 

 July, with a date of maximum on June 26, would thus seem, for 

 this year's return of its meteor-period at least, to be in a fair way 

 to be realised. A. S. Herslhel. 



Observatory House, Slough, June 20. 



Purification of Sulphur. 

 It is never a very pleasant or gracious task to reply to 

 criticism. When that criticism, however, is based on a mis- 

 apprehension of the facts, and is consequently wrong and 

 misleading, and is, moreover, enforced by whatever weight 

 " authority " may carry, its correction is simplified down to a 

 plain matter of duty. The criticism to which I refer is con- 

 tained in Prof Armstrong's address to the Chemical Society 

 [Ckeiit. Soc. /ourn., vol. xxxix. p. 1160), which has just come 

 before me. 



The passage to which I refer runs as follows. "To return 

 to sulphur, an abstract account has recently been published in the 

 Proceedings of the Royal Society (1S94, Ivi. 32) of observations 

 by Threliall, Brearley and Allen, on the electrical properties of 

 pure (sic) sulphur, i.e. sulphur from the Chance recovery pro- 

 cess purified by distillation and exhaustion in vacuo. Such a 

 process cannot be accepted by any means as an exhaustive one, 

 and it appears almost to be a case of ' love's labour lost ' to 

 apply to such material the infinite care which the authors 

 appear to have taken in making the electrical measurements. 

 Yet they arrive at the important conclusion that so long as a 

 single modification be dealt with, such sulphur does not con- 

 duct while solid. A mixture of two modifications, however, 

 does ; but in view of the possibility of changes taking place 

 during the production of the mixture, of conducting impurities 

 being introduced or generated, it is difficult to regard this latter 

 conclusion as established ; the more so as the authors in question 

 have found that, as the temperature was raised the conductivity 

 of the sulphur increased slightly up to the melting point, when 

 there was an enormous increase." 



Prof. Armstrong prefaced the paragraph which I have 

 quoted above, by some rather ungenerous remarks as to 

 the supposed attitude of physicists towards the question of 

 chemical purification of material. It is now eight years since I 

 began to endeavour to purify materials, so that I can, at all 

 eyents, agree with Prof. Armstrong on one point, viz. that it is 

 much more difficult to purify a substance up to the furthest limit 

 of chemical discrimination, than it is to determine its physical 

 properties afterwards. In the passage to which I now allude 

 — and which I do not intend to quote, for I feel sure that Prof. 

 Armstrong will, on consideration, agree with me that it is better 

 forgotten — the ' ' tongue of the scorner " is thrust out at those 

 physicists who take no thought as to the condition of material 

 examined by them. However true this may once have been, I 

 am sure that nowadays it is a mere superstition to suppose that 

 physicists, as a body, are callous on the subject of purification. 

 A great deal of modern work in this domain of chemistry has been 

 done by professed physicists ; and indeed, though I have myself 

 known several physicists enormously interested in questions of 

 purification, I have only known one chemist whose life-long 

 endeavour was to get things pure, and that was the late Prof. 



J osiah P. Cook, of Harvard. Passing to the immediate point, ■ 

 Prof. Armstrong objects to the use of the word " pure," made 

 by myself and my co-workers. With regard to this, I may say 

 that Prof. Armstrong labours under the disadvantage of only 

 having an abstract under his notice. In the paper (if it ever gels 

 published) he will find that this very matter is discussed at, per- 

 haps, too great a length, and the conclusion arrived at that the 

 word ' ' pure " ought to be kept for substances in such a degree of 

 purity that existing chemical or physical means fiiil in discover- 

 ing any foreign sub.stance. " Pure," therefore, as we have used 

 the word, has no meaning except in connection with the existing 

 state of the art of chemistry, and was adopted by us rather than 

 the word "purified," as the re.sult of some consideration, in 

 which a desire to avoid pedantry had some weight. Since Prof. 

 Armstrong refers eulogistically to Stas — as who would not — I 

 may perhaps refer him to the following passages by Stas himself 

 (Biil/eliii de i'Acaiit'iiiie Koyale des Sciences, &'c.,de Belgiijue. 

 2 series, vol. x. p. 253), than whom no one could be more 

 careful as to the use he makes of the word " pure." 



"Jusqu'ici, il n'y a que M. Dumas qui ait tente de faire la 

 synthese du sulfure d'argent. Pour determiner le rapport pro- 

 portionnel de -ses elements, il a sulfure directement I'argent 

 par du sonfre pnr qu'il faisait passer en exces." ..." En 

 suivant cette methode, j"ai fait deux series d'experiences : 

 la premiere, comprenant trois syntheses par (/^ sou/re pur axneni 

 en exces." Stas gives no details as to the preparation of his 

 " soufre pur" (which he would certainly have done had 

 elaborate precautions been takeni ; and as a matter of fact, Stas' 

 sulphur was probably far less pure than mine, for in those days 

 there was no "Chance" sulphur. So much for the word 

 " pure." 



Prof. Armstrong considers that the process of purification 

 employed by us is " by no means an exhaustive one," &c. A 

 process is, we take it, exhaustive when it exhausts the resources 

 of physics, including chemistry. Now, we spent four years in 

 trying all likely and many unlikely methods of purification. We 

 finally, by sheer good fortune, received some Chance sulphur, 

 and our methods of discrimination at once revealed to us that, 

 when dust and water were removed, it was purer than any we 

 had been able to prepare hitherto by the most elaborate means ; 

 and we adopted it as our source of sulphur in consequence of 

 this discovery. On the other hand, there are doubtless degrees 

 in the purity attained by the commercial product. The first lot 

 we received (a present from Mr. Chance) was much purer than 

 some afterwards purchased ; and with this later sample the 

 process we employed for purification would probably not have 

 been sufficient. 



With regard to the question of the adequacy of the purifica- 

 tion — for this is the important point — we arrived at a stage at 

 which no chemical means enabled us to detect any impurity 

 whatever, and the specific resistance (if one may so misu.se the 

 term for a body which does not obey Ohm's law) rose above 

 10-'' C.G.S. units ; our limit of discrimination, and probably the 

 furthest hitherto cerlainly attained. 



As we had found that the purer the sulphur the better it in- 

 sulated, as with this sulphur we could absolutely find no impurity 

 at all, and as all our means of purification (except by repetition) 

 were exhausted, we felt that we had done all that could possibly 

 be done. 



The conclusion at which we arrived — viz. that so-called 

 mixtures of crystalline and amorphous sulphur conduct, whereas 

 pure crystalline sulphur does not — seems to us to be of consider- 

 able importance, and we therefore spared no pains to assure our- 

 selves of its truth. For this purpose we prepared crystalline 

 sulphur films, ascertained their property by non-conductivity, 

 and then converted them into the conducting mixture by appro- 

 priate heating and cooling. Conversely, we caused conducting 

 "mixtures" to become non-conducting by annealing. Prof. 

 Armstrong's criticism, therefore, is wrong, and the uncertainty, 

 which he assumes, does not exist. It is as certain that crystalline 

 sulphur is at least about a million times more non-conducting, 

 than " mixed" sulphur, as that copper conducts better than glass. 

 We spent some years in assuring ourselves of the truth of this 

 proposition, and we feel that we should be shirking our philo- 

 sophical duty were we to allow this conclusion, so laboriously 

 reached, to be set aside, or rendered nugatory, by a criticism based 

 on a misapprehension, and enforced by all the weight which Prof. 

 Armstrong's utterances so rightly carry. 



I confess I am unable to reply to that part of Prof. Arm- 

 strong's criticism in which the above conclusion is supposed to 



NO. 1393, VOL. 54J 



