JUNE 10, 1915] 
NATURE 
3907 

when concentrated. As they are now being sent 
to France daily in very large numbers it is to 
be hoped that the directions of the War Office 
recently issued will be attended to and no more 
respirators of other types, either home made or 
manufactured without official sanction, will be 
sent to the Front. 
It must not be forgotten, however, that the 
efficiency of all these contrivances is limited, and 
in the event of being exposed to several successive 
doses of gas the material ought to be washed in 
clean water and then recharged with the original 
chemical mixture. A supply of liquid is now fur- 
nished to the men in the trenches. 
A question has arisen in connection with the 
idea of the possible use by the enemy of bombs 
containing poison, to be dropped by Zeppelins 
or other aircraft. In the event of such occurrence 
the best course would be in the first instance to 



Ignition 
Device 
Thermit 
Resinous 
matter 



Sketch A. 
Appearance ofan 
incendiary 
ORISS Melted 
, white 
phos: 
Valens 

SECTION OF INCENDIARY BOMB. 
(a) The bomb, as arule, is conical, of 10 in. diameter at the base, corded 
round, and has a metal handle at theapex(see A). (4) The base is a flat cup, 
on to which a pierced metal funnel is fitted, having the ignition device and 
handle fitted at the top. (c) The funnel is generally filled with Thermit, 
which upon ignition generates intense heat, and by the time of the con- 
cussion has taken the form of molten metal of the extraordinary high tem- 
perature of over 5000" F. The molten metal is spread by the concussion. 
(@) Outside the funnel is a padding of a hichly inflammable or resinous 
material hound on with an inflammable form of rope. The resinous material 
creates a pungent smoke. (¢) There is generally some melted white phos- 
phorus in the bottom of the cap, which develops nauseous fumes. (/) In 
some cases celluloid chippings are added and occasionally a small quantity 
of petrol. 
close all the windows and doors of houses in the 
neighbourhood. Should the smell of chlorine be 
perceived indoors, a cloth wetted with strong 
solution of ammonia should be waved about in 
the air. This would produce a harmless white 
smoke consisting of sal-ammoniac, and this, even 
with excess of ammonia, would produce very little 
if any damage to furniture, ete. With regard to 
wounds produced by shrapnel or other projec- 
tiles containing phosphorus, it is improbable that 
any of the element would escape combustion in the 
air. The mischief is caused by the phosphoric 
acid produced being carried into the tissues, and 
for this dressings made slightly aikaline with 
carbonate of soda should be used till all acid has 
been removed. 
The chief result of air raids so far has been 
the production of fires, and a timely warning has 
been issued by the British Fire Prevention Com- 
NO. 2380, VOL. 95] 
| 
mittee. As the committee points out, fires are 
not usually caused by the ordinary explosive 
bomb. When fire occurs it is due to open lights 
or fires in the house or to broken gas or electric 
mains. The incendiary bomb is, however, designed 
to produce a fierce fire in itself. The accompany- 
ing diagram shows the construction of one of the 
ordinary types of these missiles. 
The fumes from such bombs contain a large 
quantity of phosphoric acid, and when inhaled pro- 
duce violent coughing, but fortunately they would 
be completely stopped by a wet cloth. Thermit, 
in its ignition, produces such a high temperature 
that the timbers of a house would be certainly 
inflamed if struck, but when the first burst of 
flame is over the extension of the fire would pro- 
ceed in the manner of fires originating otherwise 
and would have to be dealt with by the fire 
brigade in the usual way. In the meantime, every 
house in a threatened area should be furnished 
with buckets and baths full of water ready drawn 
to be applied as quickly as possible, in Guantity 
as large as possible, in case of necessity. 
W. A. TILDEN. 

VISIBILITY. 
HE factors which determine the ability of the 
eye to distinguish an object from its back- 
ground are not very well known for many of the 

conditions met with in practice. Our practical 
experience has taught us that if we wish to see 
properly under any given condition, the illumina- 
tion must not be less than a certain amount, and 
we have been content to provide the necessary 
illumination without having any very clear idea 
what are the aids and hindrances to good seeing. 
A discussion on some aspects of the subject of 
visibility was initiated at the Hluminating En- 
gineering Society, on April 27, by Messrs. Pater- 
son and Dudding, of the National Physical 
Laboratory ; and many speakers made interesting 
contributions to it. 
Several fields of investigation disclose them- 
selves in connection with the subject, and are 
mainly concerned with cases in which objects 
become difficult of discernment. For instance, the 
conditions required to render troops or other ob- 
jects invisible to the enemy is a matter which at 
first sight may appear simple, but if all that is 
said is true, the problem is a complicated one, and 
would justify thorough research into it. 
It has been stated that if a donkey and a zebra 
are located in a distant field (and remained. there 
to be observed) the zebra would become invisible 
long before the donkey. This effect was discussed 
in an interesting article by Col. F. N. Maude in 
Land and Water for January 30, in which he gave 
an incident from practical experience of a regiment 
with pipeclayed belts and accoutrements being 
invisible by the side of another regiment which had 
equipment of a more uniform nature. 
The question of the visibility of distant faint 
sources of light at night is the simplest form in 

which the problem presents itself, because first, 
' the contrast between the light and its background 
