130 PROFESSOR JOHN GIBSON ON 
Sutpuuric Acip AND Formic AcID. 
The action of sulphuric acid on formic acid affords a striking example of the 
relationship between the value for y and the dehydrating power of sulphuric acid. 
Ultramaximal solutions of sulphuric acid of such concentration, that the value for y is 
considerable, readily effect the dehydration of formic acid, with production of carbonic 
oxide; but in solutions where the value for y is small this dehydrating power is also 
small, and ultimately disappears on progressive dilution as the value for y approaches 
zero, that is, as the concentration of the ultramaximal solution approaches that of the 
maximal solution, z.e. 30 per cent. H,SO,. 
The following experiments were tried: In each experiment a small quantity, 
5 c.c., in volume of anhydrous formic acid was mixed with 100 c.c. of one of a series of 
standard solutions of sulphuric acid having the concentrations given in Table I. 
The mixture was heated in a distilling flask, and when necessary a slow current of 
carbonic anhydride was used to sweep out the last portion of carbonic oxide evolved, 
this gas being collected and measured over a strong solution of caustic soda. With 
the more highly concentrated acid solutions carbonic oxide was given off freely, but 
when the concentration fell to [=8'1 (y=0'123 ohm~'c.m.~'), only a few cubic 
centimetres of carbonic oxide were given off. With '=7°3 (y=0-025), still less, 
and with I’=6:1, or lower, no carbonic oxide was obtained. (See Fig. I.) Evidently 
the sulphuric acid does not dehydrate formic acid in premaximal solutions. 
SUMMARY. 
The examples discussed so far point to a remarkable relationship between the 
velocity of very many reactions and one particular quality of the media in which they 
occur, viz. their tendency towards increase of specific conductivity, this tendency 
being measured by the value for y= K,,,x, — K. 
These examples may be summarised and correlated in the following manner :— 
Homogeneous chemical systems which undergo change either of themselves or under 
the influence of the electro-magnetic vibrations which we call “light,” change so that 
their specific electrical conductivity is increased, unless when coerced in an opposite 
direction by stronger chemical affinities. 
The justification for such an hypothesis must lie in its usefulness. It must make it 
possible to predict correctly the results of hitherto untried experiments which it 
suggests, and lead to the correlation of hitherto uncorrelated phenomena. The proof 
offered in this paper is a cumulative proof. It is drawn from a great variety of instances 
widely different in character. 
Apparent exceptions are capable of classification and correlation. 
For instance, owing to strong chemical affinities, it is impossible to prepare a solution 
containing any considerable concentration of hydrogen ions along with a corresponding 
