396 



NATURE 



[Fed. 26, 1880 



the colours displayed by the photographs obtained by Becquerel 

 and other earlier experimenters from coloured objects, were due 

 merely to the interference of the rays of light reflected from the 

 plates, that is, were the colours of turbid media. The conclusion 

 reached by me was based on a repetition in 1S53 of Becquerel's 

 experiments, on a personal examination of two of the coloured 

 photographs of Niepce, and on the results obtained by C. 

 Schultz-Sellack, fiagg. Ann. for 1S71, p. 449; and finally, in .1 

 more general way, on a prolonged photographic experience in 

 which colours were not unfrequently obtained by myself that 

 imitated those of nature, but were really due to the interference 

 of light. Ogden N. Rood 



New York, January 



[It is a matter of regret that Prof. Rood should misunderstand 

 the entire tone of a passage which occurs in my review of his 

 admirable work on "Modern Chromatics." In the passage 

 referred to I stated that there were one or two points which 

 would be better revised whenever a second edition should be 

 called for : these being statements set down without any qualifi- 

 cation whatsoever, but which are not universally accepted, and 

 which, as being still matters not removed from the field of con- 

 troversy, should not be stated without reserve in a text-book 

 where space forbids discussion or extended reference. There 

 are physiologists who do not accept without some qualification 

 Helmholtz's statement that the blue tint of eyes is simply due to 

 turbidity of the medium. The most eminent authorities on the 

 subject in this country do not accept the view that the beautiful 

 photographs in colours obtained by Becquerel and others ate 

 merely due to interference of light, in fact their opinion is the 

 very reverse. Hence, while the sentence to which I have taken 

 exception may be regarded as Prof. Rood's opinion, it cannot be 

 regarded as a universally accepted view; and that is all I have 

 desired to intimate. 



As to the experiment with the rotating disk claimed by Prof. 

 Rood, there is no doubt whatever that he has the pri 

 is, however, literally true that the experiment, which Prof. Rood 

 claims (and rightly claims) as his own, was originally described 

 in England by Mr. T. Rose in 1S61. Nothing was further from 

 the writer's intention than to charge Prof. Rood with plagiarism 

 for describing the same experiment in America in i860. — 

 b. P. T.j 



Etna 



Prof. O. Silvestri writes me in a letter dated Catania, 

 February 12: — " L'Etna dal 10 Febbrajo preseuta fenomeni 

 eruttivi dentro al cratere centrale e ci ha dato una pioggia di 

 cenere che ha ricoperto tutta la neve sul fianco Est-Sud-Est." 

 Some of the ash inclosed in the letter is grey in colour, 

 exactly like pulverised basalt. Under a high power it i_; 

 seen to consist of minute transparent tabular crystals (probably 

 felspar), mixed with greenish and brownish particles. The mud 

 craters near Paterno have lately exhibited increased activi. 

 slight shocks of earthquake have been felt on the north-east and 

 south-west sides of the mountain. G. F. Rodwell 



February 23 



Ice-Crystals and Filaments 

 If the Duke of Argyll will look again at the second of the 

 three letters in Nature, vol. xxi. p. 302, he will see that, 

 although my explanation of the ice filaments agrees on the whole 

 with those contained in the other two, it differs in one important 

 respect, and is not liable to the chief objection which he alleges 

 against the theory. I suppose the crystallisation of the water to 

 go on pari passu with its exudation at the surface of the rotten 

 wood. If the wood be saturated v> ith water the water will begin 

 to exude by expansion as scon as its temperature falls below 

 4°C, that is, before it becomes frozen. Now the temperature 

 at the surface will fall more rapidly by radiation than that within 

 by conduction. Consequently the water will for the first time 

 be subjected to a freezing temperature when it gets beyond the 

 surface. There it will be solidified, and by the coating of 

 crystals formed, help to protect the water within from freezing. 

 It may possibly be that the slight relief from pressure which the 

 water would experience on escaping from constraint when it 

 arrives at the free surface would predispose it to immediate 

 solidification. 



A very similar arrangement of crystals of salts of lime may 

 be observed occasionally to exude from plastered walls, strongly 



confirming the supposition that the water ef which the filamentous 

 crystals in the present case are composed comes from within and 

 is not deposited as a form of hoar frost from without. 



Harlton, Cambridge, February 20 O. Fisher 



I AM astonished at the Duke of Argyll's first letter (NATURE, 

 vol. xxi. ji. 274) not having received a more adequate answer 

 from nearer home. The explanation of the phenomenon in 

 question is to be found in the action of capillary attraction (as 

 stated by Mr. Ring, p. 302), together with the growth of 

 crystals by absorption from surrounding media ; in this case 

 from fog and watery vapour in the air. 



Comb-shaped masses of ice, of a decidedly fibrous structure, 

 and several inches high, are to be observed here every winter, 

 extending over wide ranges on the loose and porous soil of the 

 wooded hills near Freiburg, especially on inclined path-borders 

 devoid of vegetation. They are found most abundantly when 

 fogs have prevailed for a longer period, with the temperature 

 below freezing-point, as has been the case these last months. 

 These filamentary masses are formed at the same times and from 

 the same causes as the hoar frost on trees, grass, &C, but of 

 comse they are much more durable than the latter, being of a 

 much coarser texture, and not exposed to the destructive action 

 of the winds and of the sun's rays. They are, moreover, pro- 

 tected by grains and clusters of soil raised by the growth of the 

 filaments, and sometimes forming a covering sufficient to conceal 

 the icy masses from a superficial inspection, the structure being 

 surprisingly revealed by a stick's stroke. 



During the extraordinarily protracted frost-period of last 

 January, a snow 7 -sheet of a few inches only persisted round about 

 here for several weeks. Meanwhile we have had extremely quiet 

 air (with high barometer) and fogs of varying density, only 

 interrupted sometimes by a few hours of sunshine about noon. 

 Now a very remarkable consequence of this state of weather « a> 

 to be observed, offering, as it were, another proof for the ex- 

 planation given above. The thin snow-cover served as a soil, 

 from w hich grew up everywhere the most beautiful and delicate 

 crystalline structures, forming a superimposed stratum, in many 

 places of much greater height than the snow-crust, on which it 

 arose, but, of course, of a very loose cohesion. This, no doubt, 

 is the same phenomenon as that mentioned in the beginning of 

 the Duke of Argyll's first letter, being due, likewise, to the 

 attraction continually exercised by crystals of ice and snow on 

 the watery vapour of the cold air. The crystalline fern-growths 

 in these cases, and the fibrous masses in and upon the porous 

 soil may be considered as equivalent, the condensation of vapour 

 being more abundant, and producing fibrous instead of more 

 delicate crystalline structures, when taking place on and between 

 loose earthy (or wooden) particles. 



Undoubtedly such phenomena have been oftentimes observed 

 in many countries. A caieful and detailed description (with 

 illustrations) entitled " Uebcr Eiskry-talle in lockerem Schutte," 

 has been given by Dr. G. A. Koch in the Neues Jahrlmch 

 ralogie, 1S77, p. 449, especially considering these 

 structures from a crystallographical point of view. 



Freiburg im Breisgau, February 21 D. Wetterhax 



"Scientific Jokes" 



Will Mr. Moulton compassionate my ignorance, and explain 

 to me (and to many others equally uninformed, and equally 

 thirsting for information) in what sense it is true that " The 

 energy of heat is made up of heat and temperature." I have 

 been taught that heat is energy. If this be true, the energy of 

 heat cannot depend on temperature. 



I would also beg for an explanation of the statement that 

 " Force is the power of producing energy." I have been taught 

 that energy cannot be produced or destroyed by any natural 

 process whatever. 



As to the explanation of the earth's magnetism, I should have 

 said that Prof. Rowland was the first to imagine it (as he was 

 the discoverer of the beautiful result on which it is based), but 

 he saw at once its incompatibility with known facts. His 

 trenchant note in the Philosophical Magazine f (St last August, in 

 which he points out " more exactly" Messrs. Ayrton and Perry's 

 error, has not yet (to my knowledge) been answered. And no 

 wonder ; for an error of nearly sixty thousand million per cent. 

 is not easily got over ! G. H. 



