Fire or Acid

American physicist Benjamin Thompson wrote the following letter in 1775 at the start of the American Revolutionary War, a war during which Thompson, a loyalist, acted as a spy for the British Army. The letter actually contains two different messages. The first, longer message spans three pages, was readable by all and is notable in these pictures by its faint appearance, whilst the second, secret message – now dark – was written between the lines of the first message using invisible ink. The invisible ink would be revealed by applying either heat or a chemical solution to the paper; the letter’s creator would mark the correspondence somewhere – with an ‘F’ for Fire or ‘A’ for Acid – depending on the treatment needed to reveal its true message.


Previously Invisible Message


If you will be so kind as to deliver to Mr. of Boston, the Papers which I left in your care, and take his Receipt for the same, You will much oblige

Your Humble Servant


Saturday May 6th 1775

Decoy Message

Woburn May 6th 1775

Sir, In compliance with your desires I embrace this first opportunity that has offered since I left Boston to send you some account of the Situation of affairs in this part of the Country.

I need not trouble you with a particular account of the affair at Concord on Wednesday the 19th [ ] nor of the subsequent gathering at Cambridge [ ], as you have doubtless already better intelligence of them affairs than I am able to give you.

The only information that I can give you that can be of any consequence … lately received from a Field officer in the Rebel Army (if that mass of confusion may be called an Army) & from a member of the Provincial Congress that is now setting at Watertown. By them I learn that an Army consisting of 30,000 effective men is speedily to be raised in the four New England Governments, & that the quota for this Province is 13600. That as soon as this Army shall be raised & regulated, it is generally supposed that the first movement will be to make a feint attack upon the Town of Boston & at the same time to attempt the Castle with the main body of the Army-

Whether this will be the precise plan of operation or not I cannot determine, but really believe that the congress & their officers in General are determined to, & really imagine that they shall be masters of both the Castle & the Town of Boston in a very short time-

I am credibly informed that the Congress mean now to prosecute their plan of Independence at all adventures & in order to this that application will speedily be made to someone of the European Powers for assistance against Great Britain – And this I am the moreready to believe as I have it from a member of the congress, one who is intimately acquainted with the secrets of the party, & a man whom I can not suspect of any design either to amuse or deceive me.–

But this their plan is by no means [?] comonly known or suspecte by the People in general, but they are still fed up with the old story that “their invaluable rights & priviledges are “invaded”, & are taught to believe that the military preparations which are now making are in defence of them & to obtain redress-

As to the quantity of ordnances other military stores that have been provided by the Congress & I have not been able to obtain any satisfactory accounts. But believe that the quantity is by no means equal.

…plan of operation they have formed – …Dunbar from Canada, & Ens. Hamilton of …Reg [?] with their servants are Prisoners in this Town, But I have not permitted to see them tho’ I have made frequent applications for that purpose –

As to my own situation, it has been very disagreeable since I left Boston, as upon my refusing to bear Arms against the king I was more than ever suspected by the People in this part of the Country – And it has been with difficulty … few friends that I have here have more than once prevented my bein asassinated.

I am extremely unhappy that my confinement to this Town (by this deluded people) should put it out … power to do any thing for the good of the service But … soon to have an opportunity of giving convincing … of my Loyalty to the King, & gratitude to all my benefactors – in the mean time you will give me leave to assure you in the most solemn manner possible, that neither the threats nor promises of this wicked & Rebellious faction shall ever induce me to do any thing contrary to my professed loyalty to his Majesty – But that on the contrary I do with the greatest pleasure & alacrity dedicate my Life & fortune to the service of my rightful sovereign King George the third –

I am Sir with the greatest respect

Your much obliged and Most obedient Servant

(Name cut out)

P.S. (Name cut out) comes on purpose to bring this & the Pistol you was kind enough to lend me – I beg you would be so good as to procure him a pass to return