POPULAR SCIENCE REVIEW. 
318 
After the lapse of foul* or five hours the substance is entirely converted into 
red lead. If the lead peroxide he heated to 350° gas is plentifully evolved, 
but the action very soon ceases, and a product is obtained having exactly 
the composition of lead sesquioxide. It is a greenish-brown powder, which 
both in properties and in point of composition stands intermediate between 
red lead and lead peroxide. It can also be produced by heating lead oxide 
or, what is better, lead carbonate to 350° in a current of air or oxygen. The 
cause of the failure of the earlier chemists to isolate this oxide is clearly to 
be traced to their having employed too high temperatures. 
The Restoration of Faded Handwriting. — In a paper on several interesting 
points connected with chemical technology (“ Journ. prakt. Chem.,” 1878, 
xvii. 83) von Bibra discusses the best means of rendering legible writing 
which in i process of time has become obscure. He refers to the custom of 
•employing freshly prepared ammonium hydrosulphate, which, he states, is to 
be applied with a brush, the excess to be removed with water and the paper 
or parchment then quickly dried between folds of blotting-paper. In the 
case where an important document is to be deciphered we cannot help 
regarding this as little else than heroic treatment, and prefer to pour the 
reagent in a watch-glass and to hold the manuscript over it, exposing it to the 
fumes of the strong hydrosulphate just so long as is necessary to develop the 
handwriting. At best the method is not a good one, because it converts the 
iron of the ink into the metallic sulphide which rapidly undergoes oxidation ; 
the writing in a very short time will fade away as the sulphate is diffused 
through the moist paper, and the characters will appear blurred when any 
subsequent attempt is made to revive them. Yon Bibra has now found in a 
moderately concentrated aqueous solution of tannin (gallotannic acid) an 
agent which produces the desired result, and at the same time possesses 
none of the disadvantages of the hydrosulphate. He applies the tannin 
solution with a brush, removes the excess by a current of water, and dries 
the document at a temperature of 50° — 60° R. The writing developed in this 
manner is clear and very black, remaining so after the lapse of several 
months. These methods, it need hardly be stated, refer only to manuscripts 
where ordinary ink has been employed, and not to documents written with 
Indian or carbon ink. 
The Chemical Action of the Electric Discharge on Persulphuric Acid. — As 
this acid can be formed by the action of oxygen on sulphurous acid ( see 
Popular Science Revieiv, April, 1878, 207) it occurred to Bertlielot that it 
may also be prepared by the direct action of oxygen on sulphuric acid 
S 2 0 6 + O = S 2 0 7 . This fact he has now established ( a Compt. rend.” 1878, 
Ixxxvi. 277). He subjected 277 milligrammes of sulphuric anhydride in 
contact with 60 ccm. of dry oxygen to the action of the electric discharge for 
the space of eight horns ; the tube containing these materials was then 
heated to remove a few little drops of unaltered anhydride, and he then 
continued the transmission of the current for some hours. By this time the 
anhydride had completely disappeared, and in the place of it there was a 
substance closely resembling persulphuric acid. Rather more than 20 ccm. 
of the gas had disappeared. The new body possessed the composition of 
persulphuric acid. It was found that the anhydride and oxygen, when 
placed together, do not enter into combination unless the electric currents 
