22 Proceedings of Royal Society of Edinburgh, [sess. 
when its specific gravity is high, and when it holds in solution much 
urea and especially salts, its resistance is low, and, vice versa, where 
the specific gravity is low the resistance is high. 
To this general rule — “ that the resistance varies inversely with 
the specific gravity ” — there are certain exceptions, which can be 
arranged accordingly as they occur in acute or in chronic diseases. 
Amongst the former, acute croupous pneumonia, amongst the 
latter diabetes mellitus, are the most notable. 
That the urine of a case of acute croupous pneumonia should 
offer a higher electrical resistance than would be predicated from its 
specific gravity is easily understood, when the great diminution of 
the chlorides in the urine of a case of this disease is remembered ; 
but the increased resistance of a diabetic urine affords at first sight 
a question of a more interesting nature. 
The field of inquiry, if we include all diseases, being a very wide 
one, has been restricted in this paper mainly to the consideration of 
the resistances of the urines of people in apparent good health, and 
of those suffering from the more chronic diseases, excluding also 
local surgical affections of the urinary organs. 
Of all the diseases of this nature, the resistances of the urines of 
which I have been able to test, that of diabetes mellitus alone affords 
a tolerably constant exception to the rule already laid down. In 
this disease the specific gravity of the urine is high, and the elec- 
trical resistance offered by it is also high, and sometimes very 
high. 
According to various authorities (Professor V. Jaksch, page 256 ; 
MacMunn, pages 36 and 115, &c.), the urea and chlorides shew a 
greatly increased excretion in this disease : by this they must mean 
a total increase, and not one relative to the amount of water passed. 
From some experiments made by means of artificial solutions of 
urea, sodium chloride, sugar, &c., to ascertain the influence exercised 
by the various constituents of the urine upon the electrical resist- 
ance, it would appear that this is almost wholly dependent upon the 
amount of the salts (electrolytes) present ; if these be absent, the 
resistance is determined mainly by the amount of the urea. 
A diabetic urine offers (so far as the experiments at present made 
go) no exception to this rule ; its increased resistance is dependent 
upon the diminished amount of the salts relative to the amount of 
