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POPULAR SCIENCE REVIEW. 
appears never to be complete, a portion of the hydrogen peroxide always re- 
maining unchanged. Anhydrous persulphuric acid is most readily prepared 
by allowing the discharge to take place for eight or ten hours through the 
tubes constructed for such operations, by which time their interior is found to be 
covered with drops of a viscous liquid : at 0° it becomes solid and is crystal- 
line; in places it has a granular structure ; in others thin transparent crystals 
are seen traversing the length of the tubes ; brilliant scales are also observed 
attached to the glass. The acid resembles anhydrous sulphuric acid ; the 
latter substance, however, forms opaque crystals which are shorter, much 
finer and narrower than them. At 10° persulphuric acid has a vapour ten- 
sion of several centimetres, and sublimes spontaneously from one part to 
another of the tube in which it is confined. Its composition was determined 
both by synthesis and analysis. After the two gases had entered into reaction 
the residue was removed from the tube by means of the Sprengel pump and 
measured : if the change takes place in accordance with the following equa- 
tion the residue should be one-eighth of the gas consumed : — 
S 2 0 4 + 0 4 = S 2 0 7 + 0 
4 vols. 4 vols. 1 vol. 
In two experiments the numbers 8*2 and 7*9 were arrived at in place of the 
ratio 8*1. Four volumes of sulphurous acid and three of oxygen, therefore, 
appear to combine to form persulphuric acid : — 
s 2 o 4 + o 3 = s 2 0 7 
The acid was analysed by several methods : by using a standard solution of 
tin chloride, estimating the change by means of potassium permanganate, and 
subsequently determining the sulphuric acid as barium sulphate ; again, by 
employing a standard solution of ferrous sulphate, and afterwards observing 
the amount of a standard solution of permanganate required to complete the 
oxidation ; also by the use of potassium iodide and hyposulphite, as well as 
by the employment of sulphurous acid and potassium iodide. Each method 
gave numbers which indicated the formula S 2 0 7 as that of the new acid. 
At 0°, or at temperatures below zero, the acid may be preserved for some 
days ; it is not a stable compound, however, and after the lapse of 14 days 
it commences to spontaneously decompose ; the aqueous solution very rapidly 
undergoes change ; when it is dissolved in concentrated sulphuric acid the 
decomposition is retarded ; in this case also oxygen is slowly evolved, and at 
the end of six weeks the acid is completely destroyed. An application of 
heat causes it to separate into sulphuric anhydride and oxygen. It develops 
dense fumes in contact with air, atmospheric moisture is absorbed, and hydrated 
sulphuric acid formed. Dissolved in concentred sulphuric acid it appears, as 
has already been stated, to have the power, for a time, of resisting decom- 
position ; the solution, after the lapse of the first twenty-four hours, appears, 
from the results of volumetric analyses, to undergo no appreciable change. 
When placed in contact with baryta water a portion of the acid apparently 
escapes decomposition and barium persulphate is produced ; the salt, which 
could not be analysed, is soon converted into the ordinary sulphate, oxygen 
being evolved. Persulphuric acid does not oxidize arsenious acid or oxalic 
acid in the cold, and in this respect it closely resembles hydrogen peroxide ; 
it shows a marked difference from that compound in other reactions in that it 
