In the summer, I ran the first of a series of posts about prisoner of war mail (see here) and I briefly mentioned the censorship of PoW mail. Incoming and outgoing mail was censored by Canadian military and civilian officials to prevent PoWs from leaking sensitive information about Canadian wartime operations, their locations, and information that could be deemed harmful to the Canadian war effort.
Now PoWs were aware that their letters were being censored and were instructed not to include sensitive material in their correspondence. While many followed these instructions, a number of PoWs sought to circumvent the censorship process by hiding secret messages. Some PoWs used elaborate codes hidden within the body of their messages, some used predetermined words or messages, and others turned to more secretive methods, namely invisible inks.
I came across one of these messages that appears to have employed some sort of invisible ink in an attempt to convey a message from the spokesman of a remote lumber camp back to the spokesman at the base camp (Camp 23 at Monteith, Ontario).
The original message (in black text) is quite innocent:
Dear Felix:-
As you see, I have run out of writing paper. Have you received my letter of 20.12.?? Please acknowledge the receipt of it, so that I will know that you have been informed. I hope that everything turns out to our satisfaction.
Yes Felix, we took everything on our own heads when we left the camp — everything except injustice.
Now I have some more requests, which you will certainly fulfill for me.
I urgently need 3 arch-supports, 4 portfolios and 2 pads (jotting paper) — This sort of article is not to be had here, and you know — 30 cents.
I am enclosing a list for the exchange of clothing. Please send it here as soon as possible. When can I count on the case of books? Christmas is coming and we have nothing to read. Please see to it that the carpenters complete the case. As far as everything else is concerned, everything here is keeping with the times.
In the hope that you (plural) spend a happy Christmas, naturally wet, I remained with best wishes.–
Your comrade, Wilhelm
In the white text, the letter takes a different turn.
Interestingly, this is the first case of Anti-Semitism I have so far encountered in my (intial) research. I have yet to discover whether there were any repercussions for sending the letter but it does highlight some of the many challenges internment officials faced during the course of the war.
POWs in Canada 
Just another textbook case of why studying history is the coolest profession!