Juty 29, 1915] 
NATURE 
591 


ean in a horizontal position. Narrow strips of wood 
were nailed to the edges of the board to form a 
shallow ridge. One of these strips C was made 
adjustable. The blotting paper was moistened with 
water, and this was evaporated by blowing steam 
through the water in the can. The water vapour at 
a temperature of about 80° will stream steadily 
upwards from the surface of the can and flow over 
the board in an undulating cloud, separated from the 
surface of the board by a thin transparent layer of 
uncondensed vapour. On illuminating the cloud with 
a nearly horizontal beam of sunlight S, large patches 
of gradually changing colour appear when the eye E 
is directed slightly upwards towards the lower surface 
of the board. The beam of sunlight should be adjusted 
so that it just fails to illuminate the surface of the 
board, and the colours are best seen when this surface 
is blackened. The air around the board should also 
be quite undisturbed. These colours rival in brillianey 
those to be seen on soap films, and present the features 
of sunset colours. It appears, then, that some of the 
brilliancy and extensiveness of sunset colours is due to 
a quiescent state or regular motion of the clouds or 
mist at sunset, and also to a distribution into layers 
of droplets of nearly uniform size. 
F. W. Jorpan. 
South-West Polytechnic, Chelsea, S.W. 

Non-Poisonous Character of Nitroglycerin. 
WitH regard to the dose of nitroglycerin referred 
to in the notice of a book in Nature of July 22 
(p. 560), it may be useful to have the facts correctly 
stated. The reference was clearly to a passage in the 
“Extra Pharmacopeeia” (sixteenth edition, vol. i., 
P- 527), in which I say :—‘‘ An employé in the author’s 
laboratory (1905) ate a piece of the nitroglycerin mass 
weighing about 2 oz., mistaking it for ordinary chocolate. 
A bad headache supervened, necessitating his lying 
down, but he was at work again on the following 
day.” 
The ‘‘mass’’ in question is composed of nitro- 
glycerin with chocolate in the proportion of 1/100 grain 
in 2} grains; the amount of nitroglycerin consumed 
therefore by the predatory individual on that occasion 
Was approximately 33 grains. 
May I add that the young man was a German 
apprentice of mine, and that his exclamations in half- 
broken English, to the effect, ‘‘Mein Gott, I ‘shall 
die, I shall die!” as he gavotted round the laboratory 
waving his arms about, were the cause of some mirth 
to bystanders. As things have turned out he possibly 
has died by now from the effects of nitroglycerin 
employed in another way. 
Considering the powerful vasodilator action of this 
and allied drugs (the late Prof. Leech determined that 
the circulation is distinctly affected by even 1/1000 
grain of nitroglycerin), it is of interest to realise to 
what a remarkable extent they are tolerated. Single 
doses of 5 grains and daily doses of 20 grains have 
been administered medicinally with safety, according 
to the Brit. Med. Jl., Epitome ii., 1905, p. 52. Has 
any one of your readers any knowledge of higher 
amounts having been talxen ? 
W. H. MartinDAace. 
10 New Cavendish Street, London, W. 
Mr. MartTINDALE’s extremely interesting letter sup- 
plies the clue as to how anyone could mistake nitro- 
glycerin for chocolate, but as the book referred to 
gives no reference and omits the word “‘mass”’ after 
nitroglycerin, the ordinary reader will gain a some- 
what confused idea as to the toxic action and char- 
acteristics of nitroglycerin. Tue REVIEWER. 
NO. 2387, VOL. 95] 
The Principle of Similitude. 
In Nature of March 18, Lord Rayleigh gives this 
formula, h=«xa6.F(avc/«), considering heat, tempera- 
ture, length, and time as four ‘“‘independent”’ units. 
If we suppose that only three of these quantities are 
‘“‘really independent,’ we obtain a different result. 
For example, if the temperature is defined as the mean 
kinetic energy of the molecules, the principle of simili- 
tude allows us only to affirm that h=xa0.F(v/Ka?,ca*). 
D. RiaBsoucHINnsky. 
Aerodynamic Institute, Koutchino. 

Structure of Hailstones. 
Wir reference to the particularly violent hailstorm 
which passed over S.E. London on Saturday afternoon, 
July 24, I observed at Woolwich that the hailstones, 
apart from being very large, had a common shape and 
structure which may perhaps be worth recording. 
All the stones examined were either oval or pear-shaped, 
but not of uniform size; the broad half consisted of 
clear ice, while the other half was uniformly opaque 
or closely stratified with alternate layers of clear and 
opaque ice. S. L. ELBorne. 
77 West Park, Eltham, S.E., July 26. 
COTTON AS A HIGH EXPLOSIVE. 
A! the recent meeting of the Society of Chemi- 
cal Industry held at Manchester, Mr. W. F. 
Reid is reported to have made the statement that 
nitrated cotton is not a high explosive, though 
every chemist knows that it is the typical high 
explosive. The fact that certain newspaper 
writers have differentiated between nitrated cotton 
and nitrated benzene or toluene, or any other coal- 
tar derivative, has nothing to do with the differ- 
entiation of a high explosive (which is of itself 
nitrated, and contains within itself sufficient 
oxygen to allow of its explosion) and those 
mechanical mixtures, such as gunpowder, which 
have been now superseded. A letter from Sir 
William Ramsay published in the Times of July 19 
makes all these matters perfectly plain, and no 
responsible person would dispute them. I was 
present in the House of Lords when Lord Charn- 
wood brought his statement before that House, 
and I also heard the rest of the debate, including 
the answer of the Marquess of Crewe. The 
‘House, consisting of those who are necessarily 
laymen so far as their chemical knowledge is con- 
cerned, found some difficulty in following the 
arguments as to whether any substitute for cotton 
could be effectively used. 
To the chemist the matter is perfectly plain, and 
it has been stated with some degree of precision 
in an article which appeared in Nature of July tf. 
It is true, and has been admitted from the very 
start of what is now known as the “Cotton Cam- . 
paign,” that some form of nitrated cellulose can 
be made from anything which contains cellulose. 
There is not the slightest difference of scientific 
opinion on this matter, and every competent 
chemist will concur with what I say, but in prac- 
tical matters things stand on a totally different 
footing. There must not only be a regularity of 
the supply of material, but there must be uni- 
formity of quality; and, in the article already 
| referred to, this point has been made tolerably 


