SCIENCE AND INDUSTRY. 
the reason for the presence of rather ugly coloured diagonal stain 
marks on their letters. These were, of course, the effects of the testing 
solutions used by the German censors. 
Strangely enough, the German methods were usually clumsy and 
often inefficient, besides which they left the letter in a very soiled con- 
dition. The British chemists evolved adequate tests, and after the letter 
had been treated, the stain marks were usually removed, the letter was 
washed and ironed, and then sent on to its destination almost in its 
original state. Some of the chemists could probably give many laun- 
dresses a tip or two as to capable work. ; 
There are some inks which are developed by purely physical, as 
opposed to chemical methods. These at first gave great promise, but 
they are not used much now, for the majority of them are so active that 
it is rather easy to detect them. 
A solution of a salt of radium was experimented with. This can 
be used at almost infinite dilution, say one part of the salt in 10,000,000 
of water. Now the majority of radium salts are colourless, and in any 
case the great dilution precludes the use of purely chemical development. 
One of the chief properties of radio-active substances, however, is their 
ability to affect a photographie plate, and hence, to get a development, 
it was only necessary to place the sheet on which the message was written 
over a photographie plate enclosed in a light tight paper bag. After 
a space of, say, eight hours—more or less, according to the concentra- 
tion of the solution used—the plate was developed in the ordinary way, 
The use of radio-active solutions failed, however, because it is very easy 
to determine their presence. It is only necessary to bring the suspected 
sheet near a charged gold-leaf electroscope. If any radio-active sub- 
stance is present, the leaves of the electroscope fall, owing to the ionizing 
action of the radiations. 
Certain substances show marked fluorescence when exposed to ultra- 
violet light rays, and this property has found limited application in 
secret communication. Cases have also been known where a postcard 
has been split in two, a message written in a substance of high density, 
and the pieces reunited. When exposed to X-rays, a clear shadow of the 
message is seen. 
There are quite a number of other aspects of the work, but probably 
enough has been written to indicate that the problem of secret communi- 
cation is a Complex and fascinating one. 
