December 19, 191 2] 



NATURE 



437 



deposit consists of a multitude of small lenses 

 easily seen with a hand magnifier. In the track 

 of the flame or sulphuric acid the lenses are 

 larger, often passing' into flat masses which, on 

 evaporation, show the usual colours of thin plates. 

 When the glass is seen against a dark ground, 

 and is so held that regularly reflected light does 

 not reach the eye, the general surface shows 

 bright, while the track of the flame or acid is by 

 comparison dark or black. It will be convenient 

 thus to speak of the deposit as bright or dark — 

 descriptive words implying no doubtful hypothesis. 

 The question is what difference in the glass surface 

 determines the two kinds of deposit. 



In Aitken's view (Proc. Ed. Soc, p. 94, 1893; 

 Nature, June 15, 191 1), the flame acts by the 

 deposit of numerous fine particles constituting 

 nuclei of aqueous condensation, and in like 

 manner he attributes the effect of sulphuric (or 

 hydro-fluoric) acid to a water-attracting residue 

 remaining in spite of washing. On the other 

 hand, I was disposed to refer the dark deposit to 

 a greater degree of freedom from grease or other 

 water-repelling contamination (Nature, May 25, 

 191 1 ), supposing that a clean surface of glass 

 would everywhere attract moisture. It will be 

 seen that the two views are sharply contrasted. 



My first experiments were directed to improving 

 the washing after hot sulphuric or hydrofluoric 

 acid. It soon appeared that rinsing and soaking 

 prolonged over twenty-four hours failed to abolish 

 the dark track ; but probably Mr. Aitken would 

 not regard this as at all conclusive. It was more 

 to the point that dilute sulphuric acid (i/io) left 

 no track, even after perfunctory washing. Rather 

 to my surprise, I found that even strong sulphuric 

 acid fails if employed cold. A few drops were 

 poured upon a glass (^-plate photographic from 

 which the film had been removed), and caused to 

 form an elongated pool, say, half an inch wide. 

 After standing level for about five minutes — longer 

 than the time required for the treatment with hot 

 acid — the plate was rapidly washed under the tap, 

 soaked for a few minutes, and finallv rinsed with 

 distilled water, and dried over a spirit lamp. Ex- 

 amined when cold bv breathing, the plate showed, 

 indeed, the form of the pool, but mainly by the 

 darkness of the edge. The interior was, perhaps, 

 not^ quite indistinguishable from the ground on 

 which the acid had not acted, but there was no 

 approach to darkness. This experiment may, I 

 suppose, be taken to prove that the action of the 

 hot acid is not attributable to a residue remaining 

 after the washing. 



I have not found any other treatment which will 

 produce a dark track without the aid of heat. 

 Chromic acid, aqua regia, and strong potash are 

 alike ineffective. These reagents do' undoubtedly 

 exercise a cleansing action, so that the result is 

 not entirely in favour of the grease theory as 

 ordinarily understood. 



My son, Hon. R. J. Strutt, tried for me an ex- 

 periment in which part of an ordinarilv cleaned 

 glass was exposed for three hours to a stream of 

 strongly ozonised oxygen, the remainder being 

 NO. 2251, VOL. 90] 



protected. On examination with the breath, the 

 difference between the protected and unprotected 

 parts was scarcely visible. 



It has been mentioned that the edges of pools 

 of strong cold sulphuric acid and of many other 

 reagents impress themselves, even when there is 

 little or no eff'ect in the interior. To exhibit this 

 action at its best, it is well to employ a minimum 

 of liquid ; otherwise a creeping of the edge during 

 the time of contact may somewhat obscure it. 

 The experiment succeeds about equally well even 

 when distilled water from a wash-bottle is substi- 

 tuted for powerful reagents. On the grease 

 theory the effect may be attributed to the cleansing 

 action of a pure free surface, but other interpreta- 

 tions probably could be suggested. 



Very dark deposits, showing under suitable 

 illumination the colours of thin plates, may be 

 obtained on freshly-blown bulbs of soft glass. It 

 is convenient to fill the interior with water, to 

 which a little ink may be added. From this ob- 

 servation no particular conclusion can be deduced, 

 since the surface, though doubtless very clean, 

 has been exposed to the blow-pipe flame. In my 

 former communication, I mentioned that no satis- 

 factory result was obtained when a glass plate 

 was strongly heated on the back by a long Bunsen 

 burner; but I am now able to bring forward a 

 more successful experiment. 



A test-tube of thin glass, about h inch in 

 diameter, was cleaned internally until it gave an 

 even bright deposit. The breath is introduced 

 through a tube of smaller diameter, previously 

 warmed slightly with the hand. The closed end 

 of the test-tube was then heated in a gas flame 

 urged with a foot blow-pipe until there were signs 

 of incipient softening. After cooling, the breath 

 deposit showed interesting features, best brought 

 out by transmitted light under a magnifier. The 

 greater part of the length showed, as before, the 

 usual fine dew. As the closed end was approached 

 the drops became gradually larger, until at about 

 an inch from the end they disappeared, leaving 

 the glass covered with a nearly uniform film. One 

 advantage of the tube is that evaporation of dew, 

 once formed, is slow, unless promoted by suction 

 through the mouth-tube. As the film evaporated, 

 the colours of thin plates were seen by reflected 

 light. Since it is certain that the flame had no 

 access to the internal surface, it seems proved 

 that dark deposits can be obtained on surfaces 

 treated by heat alone. 



In some respects a tube of thin glass, open at 

 both ends, is more convenient than the test-tube. 

 It is easier to clean, and no auxiliary tube is re- 

 quired to introduce or abstract moisture. I have 

 used one of 3/10 in. diameter. Heated locally 

 over a simple spirit flame to a point short of 

 softening, it exhibited similar effects. This easy 

 experiment may be recommended to anyone inter- 

 ested in the subject. 



One of the things that I have always felt as a 

 difficulty is the comparative permanence of the 

 dark tracts. On flat plates they mav survive in 

 some degree rubbing by the finger, 'with subse- 



