NATURE 



[July 6, 191 1 



in denudation and deposition, the above factors 

 may be verified and allowed for in making an estimate of 

 the antiquity of sediments. 



limale here given is probably too low in the light 

 of these corrective factors, and it is interesting to notice 

 how much more closely it agrees with the results of the 

 totally independent method based on radio-activity than do 

 those deduced from the facts of sedimentation in the usual 

 way. Arthur Holmes. 



Mosuril, Portuguese East Africa, May 6. 



Breath Figures. 



The two interesting letters on breath figures by Lord 

 Rayleigh and Dr. Aitken (Nature, May 25 and June 15) 

 seem to me to contain a statement of the cause of this 

 phenomenon as well as the data necessary to support it. 



Thus it is shown that a blow-pipe flame, burning 

 sulphur, sulphuric acid, hydrofluoric acid, and caustic soda 

 give these breath figures, while heat and alcohol flame give 

 no such result. The conclusion apparent from these 

 chemical data is that when the glass is coated with a 

 film having an affinity for water, breath figures are 

 formed. 



Coal gas contains sulphur, and a blow-pipe flame gives 

 sufficient sulphuric acid to form a film on giass ; burning 

 sulphur gives similar acid products, and both yield breath 

 figures. 



Sulphuric acid, hydrofluoric acid, and caustic soda are 

 each capable of dissolving glass, which implies wetting 

 and a certain amount of penetration ; washing does not 

 immediately remove this, and a film of acid or alkali is 

 left capable likewise of forming breath figures. 



In ammonia solution we have a strong alkali which 

 cannot dissolve glass in the caustic soda sense; when it 

 is allowed even to stand on a glass plate no breath figure 

 is formed, but when it is well rubbed in a faint figure is 

 produced. 



If breath figures, from blow-pipe flames, say, be soaked 

 in ammonia solution and washed, they may be gradual!} 

 destroyed — by neutralisation of the acid in the superficial 

 pores of the glass — until breath outlines only exist. These 

 lines correspond to the lines of greatest acid penetration, 

 and would be represented by charred lines on a piece of 

 wood. 



This gradual destruction of the figures on gradual 

 neutralisation of the acid conclusively shows that these 

 figures are neither due to cleanliness nor dust, as has been 

 suggested. 



This explanation enables one to predict that Dr. Aitken's 

 suggested experiment of burning pure hydrogen in dust- 

 less air would give breath figures, while pure (dusty) 

 hydrogen burning in pure (dusty) oxygen would give no 

 figures, the reasons being that pure hydrogen burning in 

 air gives sufficient nitric acid to produce figures, while 

 pure hydrogen burning in pure oxygen produces no acid. 

 and would produce no figures. 



Caeteris paribus, it may be inferred that pure quartz 

 glass would not give figures with sulphuric acid, but with 

 hydrofluoric acid and caustic •soda. 



If the rays from radium can produce breath figures on 

 • onstitutes another 1 au >e. 



Glasgow. George Craig. 



A Zenith Halo. 



W11.1. you permit me to quarrel with your correspondent 

 for the heading "A Zenith Rainbow," attached to his 

 letter from Bruges, pul.li-.ln'!! i,, Naturi of May 11, 

 Thi jilirnomenon described was not a rainbow, 

 as Mi. Gold has taken pains to point out. The heading 

 is unfortunate, for two reasons: first, because it tends to 

 confirm the prevalent misuse of the word "rainbow," 

 and, secondly, because it will probably lead to the 

 improper classification of Mr. Kreyer's letter in biblio- 

 graphies. 



The terminology of atmospheric optics is in a state oi 

 dire confusion, even among scientific men, but all the 

 latter ar agreed in calling the phenomenon in question a 

 halo. Mr. Gold follows Pernter and most other writers 



NO. 2175, VOL - 8/] 



in terming this particular halo an " arc of contact."' 

 However, this name, as well as the common alternative, 

 "tangent arc," is objectionable, for the reason that the 

 halo thus designated is by no means always in contact 

 with, or tangent to, the halo of 46 (or the position which 

 the latter would occupy if present). On this subject see 

 M. Besson's article " Le halo du 21 decembre 1910; un 

 arc tangent qui n'est pas tangent," in La Nature of March 

 11, 191 1, p. 248. In the picture that accompanies M. 

 Besson's article, the " tangent " arc is shown separated 

 from the halo of 46° by an interval of about 3°. 



Another common name, " circumzenithal arc," is open 

 to the objection that this halo is but one of many that ate 

 central at the zenith. 



The only accurate and distinctive name for the pheno- 

 menon is " upper quasi-tangent arc of the halo of 46°." 



Statistics of the frequency of the various halo pheno- 

 mena are misleading. Mr. Gold states, on the authority 

 of Pernter, that the arc in question had been observed 

 only about seventy times up to 1883. Besson, " Sur la 

 theorie des halos," records m observations of it in ten 

 years (1898— 1907) at Montsouris alone. If systematic 

 observations of halos were made all over the world, the 

 frequency of such phenomena would doubtless be found to 

 be far greater than is now generally supposed. 



C. FlTZHUGH TALMAN. 



U.S. Weather Bureau, Washington, May 22. 



Probably no one will be inclined to dispute Mr. 

 Talman's proposition that systematic observations would 

 largely increase the apparent frequency of the phenomenon 

 mentioned. 



With reference to the terminology, it is, as he points 

 out, unfortunate that the terms "arc of contact," 

 " tangent arc," should have come into general use for a 

 bow which is not always in contact with the halo. I can- 

 not, however, agree that Mascart's term, " quasi-tangent 

 arc," is a satisfactory substitute. It was, I believe, in- 

 tended to meet those cases when the arc is present at 

 approximately 46° from the sun, but without the 46° halo. 

 It does not fit cases for low or high solar altitudes when 

 the arc is more than 46° from the sun. I think it would 

 be better, instead of trying to indicate all the peculiarities 

 of the phenomenon by its name, to use a term such as 

 " auxiliary arc," if the present names are to be abandoned. 



The phenomenon is described by Bravais as " un 

 veritable arc-en-ciel, " and this may account for the less 

 appropriate use of the term " rainbow." 



E. Gold. 



Meteorological Office, South Kensington, 

 London, S.W., June 2. 



Jelly Rain. 



On the morning of Saturday, June 24, the ground here 

 was found to be covered with small masses of jelly about 

 as large as a pea. There had been heavy rain on Friday 

 night, and it was raining at 7 a.m., when, so far as I 

 can ascertain, the phenomenon was first seen. On being 

 examined microscopically the lumps of jelly turned out to 

 contain numerous ova of some insect, with an advanced 

 embryo in each. The egg itself is very minute — an 

 elongated oval 0-04 mm. in length. Yesterday and the day 

 before many larva? emerged, and were obviously those of 

 some species of Chironomus, though colourless, having no 

 haemoglobin, as is the case with the larvae of C. plumosus. 

 Not being an entomologist, I am at a loss to understand 

 how these egg-masses could have appeared where they did 

 unless they were conveyed by the rain, as it does not seem 

 likely that the midges would have laid their eggs on pave- 

 ments, gravel paths, tombstones, &c, even had they been 

 wi 1 ; nor has any large number of adult insects been seen 

 in the locality. It would be interesting to hear whether 

 the same thing was observed elsewhere, and whether the 

 phenomenon often occurs. Showers of alga;, small snails, 

 and even frogs have been recorded from time to time, but 

 I cannot recall a like instance to the above. 



Eton, Bucks, June jo M. D. Hill. 



