- If you dip your pen in the juice of a lemon, or of an onion, or in your own urine, or in Spirits of Vitriol and write on clean paper whatever you intend, it shall not be discerned till you hold it to the fire, and then it will appear legible.
- And, if with any of the aforementioned you write on your skin, as on your arm and back of your hand, etc, it shall not be seen till you burn a piece of paper and with the ashes rub on the place and then it will appear very plain; and this I have experienced and try'd, and therefore can say Probatum est.
- Another way is when you write a letter that you intend it shall not be discovered, but to those you think fit first to write your thoughts on one side of your letter with black ink, as usual, (but it ought to be on thin paper) and then on the contrary side, go over the said matter that you would have secret, with a clean pen dipp'd in milk and that writing shall not be read without holding it to the fire, as mentioned above, and then it will appear legible in a bluish colour.
- Or, you may write to your friend in proper sense with common ink, and let the lines be at so commodious a distance, that what you intend to be secret may be written between them with water, wherein galls have been steeped a little time (but not long enough to tincture the water) and when dry nothing of the writing between the said lines can be seen, but when it is to be read, you must with a fine hair pencil dipp'd in Copperas Water, go between the said lines and so you make it legible.
- Note: this way will give no ground for suspicion, because the letter seemeth to carry a necessary sense in those lines that are set at such a proper distance...
So, Secret Agent Knatolee - you have no excuses now.