JUNE 10, 1915] 
NATURE 
411 


do no more than stimulate mitosis in a healthy cell 
will suffice to destroy the less stable cancer cell. The 
latter is, indeed, so unstable towards the ionising 
effects of the rays that a very small dose will arrest 
development, and even cause the destruction of the 
cell. A tube guaranteed to contain five milligrams of 
pure radium bromide was several times applied to 
cases of cancer in this city, the tube being screened 
with thin sheet-lead and applied externally. It was 
afterwards found that the tube contained but 08 
megrm. of radium element. This was, therefore, a 
very small dosage. All the results obtained were, 
however, beneficial. The whole subject is probably in 
its first stages of investigation in spite of the work 
which has been done. 
Failing the guidance which investigation will assuredly 
one day give us, it is interesting, and, possibly, impor- 
tant to discuss the cell as a photo-sensitive molecular 
system, and in so far comparable with another photo- 
sensitive molecular system, the study of which is 
less difficult to pursue.* 
Of all photo-sensitive systems with which we are 
acquainted the photographic film is at once the most 
accessible to observation and the best understood; 
although in this, no more than in any other case, 
is our knowledge complete, or our views always 
capable of actual demonstration. We know it to con- 
sist of halogenised molecules emulsified in an organic 
colloid, the relations of salt and colloid being probably 
complex, and such that they react one upon the other 
in responding to the photo-electric effect. | Certain 
features in common with the cell will be recognised 
in this statement. It is, indeed, possible that we 
might apply it word for word as a general description 
of the activity of the cell as a photo-sensitive system. 
The photograph is an effect of photo-electric activity. 
This activity, which operates during exposure, 
generates the latent image. The latent image is 
afterwards acted on by the developer, and the negative 
produced. 
In this process we start with a halide of silver, 
loosely combined with the complex molecule of the 
gelatine, or in a state of solid solution, the instability 
of the silver halide being increased by its immersion 
in the gelatine according to principles which have 
been pointed out by Sir J. J. Thomson, in the case of 
ordinary solutions in water. We end up with 
separated metallic silver. The process is, then, one of 
reduction on the whole. But it is effected in two 
stages. First, the photo-electric action; then the 
chemical action of the developer or reducing agent on 
the latent image. 
The view that the process of formation of the latent 
image is founded in the expulsion of the electron 
under the photo-electric force is supported by many 
circumstances.'° . . . 
What may be described as a state of static ionisation 
is set up, the discharged electrons creating negative 
ions by attachment to the gelatine surrounding the 
silver-bromide-gelatine systems, and forming an 
electro-negative region which may be regarded chemic- 
ally as comparable to an increase in negative ionic 
concentration. 
The developer acting at this stage—i.e. when the 
latent image has been formed—finds the process of 
reduction facilitated by these conditions. For the 
developer is essentially a reducing agent, and the 
latent image represents a temporary release of the 
positive silver ion from attachment to the chlorine ion, 
the latter having lost its charge. 
The latent image is, in short, one stage in the re- 
8 Joly, Proc. Roy. Soc., Ser. B, vol. Ixxxviii., 1914. 
9 J. J. Thomson, PAzl. Mag., vol. xxxvi., 1893, P- 320. 
W Joly, NaTurE, vol. lxxii., July 27, 1905, p. 308. 
NO. 2380, VOL. 95| 

duction of the complex silver bromide molecule. But 
it is a stage reached by physical means, and owes its 
stable character to the solid nature of the medium in 
which it is immersed. Nevertheless, it runs down in 
, course of time and disappears; the negative electron 
gradually being attracted back to the central positive 
system, and re-combining with the chlorine whence 
it came. The process of destruction may be accelerated 
by over-exposure to light, X or y rays. This is the 
phenomenon of reversal or solarisation. The probable 
explanation is that by continued exposure the electro- 
static stress set up by electronic segregation accumu- 
lates to the point of rupture, and there is a hurried 
return of the electron to its starting point under the 
electric stress. 
The latent image may also be induced by friction, 
pressure, or, generally, by mechanical irritation of the 
film. The action of the mechanical stimulus is probably 
to induce directly the separation of electrons, i.e. to 
promote the negative ionisation. A latent image so 
formed can be reversed by radiation. 
The reversal of the latent image may, in some cases 
at least, be effected by the infra-red and the heat 
waves. This might have been anticipated from the 
theory given above, for we would expect a feeble 
displacement of the electrons by the long waves, under 
which effect they would re-combine. 
We have now to consider the formation of the latent 
image by chemical means. 
If the latent image is essentially the result of a 
partial reduction of the silver halide we should expect 
its creation by reaucing agents. The developers and 
sensitisers, in point of fact, generally create the latent 
image, and act most effectively when alkaline, e.g. 
pyrogallol, gallic acid, tannin, aqueous solutions of 
nicotine, and—it is stated*'—alkaline solutions of 
lactose and glucose. These substances, acting as 
developers, must, whatever stages may intervene, ulti- 
mately neutralise the charge upon the positive silver 
ion in the emulsion, setting it free as metallic silver. 
The effects are probably continuous with those of 
radiation in creating the latent image. Dilute solu- 
tion of ammonium hydrate alone will give the latent 
image. Some substances act as sensitisers, or owe 
their special efficiency as developers to their active 
absorption of the halogen. 
As opposed to the effects of the concentration of 
the negative hydroxyl ion in alkaline development, 
the action of the positive hydrogen ion upon the latent 
image is to inhibit its growth, or reverse it if already 
formed by light or otherwise. This result is made 
apparent by introducing a very weak solution of a 
mineral acid. The oxidation of the reduction product, 
or the re-halogenisation of the partially reduced silver, 
may be involved, according to the nature of the acid 
used. The feature common to all acid intervention 
is the increased concentration of the positive hydrogen 
ion. 
The photo-sensitive molecular system of the film can 
exist in different states of sensitiveness, ranging from 
a highly sensitive to a relatively insensitive state. The 
behaviour of the “ripened’’ emulsion of the fast plate 
(i.e. of an emulsion which has been subjected to a 
process of prolonged heating) is similar in kind to 
that of the ‘‘unripened’’ emulsion of the slow plate, 
but in the former all the phenomena are relatively 
advanced. The latent image is sooner formed under a 
given exposure, and much more readily reversed. 
Chemical effects are correspondingly accelerated. The 
grain of the sensitive or ripened film is much coarser 
than that of the slow or unripened film. 
We now turn to the living cell. 
We find that radiation may, if carefully modulated, 
11 Meldola, ‘‘ Chemistry of Photography” (Macmillan, 1891), p. 190. 
