244 
SCIENCE. 
answerable, * * be was highly esteemed by the independent 
members of the profession at large in Germany. The last 
meeting of the * German Apothecaries Association' which he 
attended, reading the inaugural address, was made 
the occasion of quiet and impressive ovations 
which the modest man received with deep feel- 
ing. The kind and respectful regard with which the 
venerable scientist was treated by almost all the members 
present, was often referred to by him in his family-circle 
with pride and satisfaction during the twelve months which 
were still allotted to him. He felt at that day, if ever, that 
he had not lived in vain, and that the seed which he had 
sown would not be lost, but that coming generations would 
yet profit from and be benefited by it. 
Besides the works mentioned, he wrote the following : 
The Mechanical Theory of Chemical Affinity , etc. f General 
Theory of Motion and Force as a basis of Physics and Che- 
mistry A Manual of volumetric Analysis f edited five times, 
and last, but not least, a History of the Earth; Geology 
founded on a new basis f which was edited twice. 
Of the various improved analvtical methods devised by 
him may be mentioned the proposal to use oxalic instead 
of sulphuric acid (Gay-Lussac), to determine ti e relative 
proportion of alkalies and acids contained in a salt ; his 
combination: Sodic Carbonate against Iodine-solution; 
or, better still, Sodic Hyposulphite against Iodine-solu- 
tion, and his beautiful determination of Chlorine, by the 
use of Argentic-Nitrate-Solution, with Potassium Chro- 
mate as indicator ; of the man)’ instruments invented by 
him, the self-acting stirring apparatus, with clock-work 
arrangement, a pill-machine, an apparatus for prepaiing 
infusions by the use of steam, another for extraction by means 
of ethers, and his improved burette, with compressing 
faucet. His Manual of Volumetric Analysis in which these 
devices and many others of like importance are described, 
is considered one of the first standard works in the domain 
of analytical chemistry, and has been translated into various 
languages. 
The attempt, first made by him in his Mechanical Theory 
of Chemical Affinity, to promulgate the theory that chemical 
affinity is a mode of motion, inherent in matter, and is 
measurable only in so far as we can measure the heat that 
is liberated and bound up during the union or separation 
of two elements, is one of his greatest efforts. Liebig f him- 
self always valued this work very highly, and it is certainly 
one of the most brilliant manifestations of Mohr’s genius, 
as will be seen from the following extracts: 
“The union of two bodies by combustion always liber- 
ates a certain portion of this motion which appears in the 
shape of heat. Another portion remains in the product of 
combustion. We ate only able to measure the former, not 
the latter, and even of the former we are unable to sav how 
large an amount is due to one body and how much to the 
other. If one gramme of hydrogen unites with eight of 
oxygen, 34,462 units of heat are liberated. These indicate 
the amount of motion which both gases contained when 
yet ununited, as compared with the water resulting from 
the union. In the latter itself there is yet a certain amount 
of motion, as its liquidity and its proneness to vaporization 
1 Mechanische Theorie der chem. Affinitiit ; Braunschweig, Fr. Vieweg 
& Sohn ; 1868. 
- Allgemeine Theorie der Bewegung und Kraft, als Grundlage der 
Physik und Chemie; Braunschweig, Fr. Vieweg & Sohn ; 1869. 
3 Lehrbuch der chemisch-analytischen Titrirmethode ; Braunschweig, 
Fr. Vieweg & Sohn ; 1855 (1877). 
4 Geschichte der Erde ; eine Geologie auf neuer Grundlage ; Bonn, M. 
Cohen & Sohn ; 1866 (1875). 
* Many of the suggestions contained in his earlier works were made 
use of in the ’ Pharm . Germ.' by the very men who were his life-long 
enemies, because these and other view, were freely and sometimes -harply 
expressed by Mohr. Many of his original ideas on other subjects than 
Pharmaceutics , especially those on Geology . were also confirmed by later 
evidence ; but this equally did not serve to reconcile his opponents in 
this line of research which comprised nearly all ‘‘official geology ’ in 
Germany. 
f At the eve of its publication, the great chemist wrote to Mohr as 
follows : 
“ I am impatient to see your new book, for you seem to have treated in 
it of nearly everything that we need to know something definite about, in 
order to make chemistry a real science ; nobody as yet has really had a 
clear conception of affinity ; we simply stuck to facts, that was all. It has 
been just so with the melting-po nt, the gaseous condition, the boiling- 
point, etc.” 
successfully prove. From the oxygen contained in the 
water we are able to liberate yet another amount of heat by 
uniting it with potassium or with zinc, because potassa 
and zinc oxide are more apyrous than either potassium or 
zinc. Now, it is impossible to know what portion of the 
34,462 units of heat comes from the hydrogen, and which 
from the oxygen, and furthermore, what amount there is 
yet in the water. Therefore, we cannot reduce to an abso- 
lute measure this property of chemical affinity, only the por- 
tion that is liberated in the shape of h at. 
“This example also shows how enormously more efficient 
the motion that bodies contain as chemical affin'tv, is than 
that which they contain as heat. The water produced (9 
grammes) contains 9 u. of h., while the mixture of both 
gases before union, contained only 2'f u. of h. (the specific 
heat of hydro-oxygen-gas being o 25). But, since there 
was a development of 34,462 u. of h. during their union, by 
combustion, it follows that the motion existing in the mix- 
ture as chemical affinity, is 15,316 times that contained in it 
as heat.” 
Again : 
“ If iron develops heat while oxidizing, the dense condi- 
tion of the oxygen in the resulting ferric oxide is certainly 
an effect due to chemical affinity, but the potential energy 
of the oxygen is no longer found in the oxide ; it has been 
separated. The heat liberated during the combustion of 
the iron in oxygen-gas is the surplus of motion which iron 
and oxygen contain more than Ferric Oxide.” 
And again : 
“ If carbon and oxygen unite to form carbonic acid, there 
is no change of volume, and so it is with a mixture of 
chlorine and hydrogen. Their specific gravity after chemi- 
cal union is the same as before such union took place, be- 
cause their volume remains unchanged ; yet a great amount 
of heat has been liberated. 
“ Thus it is not true, as has been formerly assumed, that 
we may compare — for the purpose of measurement — chemi- 
cal affinity to mechanical force by calculating the amount 
of force necessary to compress a mixture of gases so as to 
give it the density possessed by the product resulting after 
chemical union has taken place. (Hydro-oxvgen-gas 
against water.” 
The quintessence of this unique volume is contained in 
these two theses : 
1. Loss of chemical affinity or liberation of heat always 
means : Higher specific gravity , higher melting point , higher 
boiling point , insolubility , chemical indifference , rigidity and 
development of but little heat on combustion. 
2. Increase of chemical affinity and absorption of heat 
always mean : Lower specific gravity, lower melting point, 
increasing solubility, proneness to chemical combination, soft- 
ness, development of much heat on combustion. 
Mr. Carl Mohr, in a biographical sketch of his father’s 
life and works, says of them :* 
“These two axioms comprise almost the whole range of 
chemical processes, and they are a mechanical theory of 
chemical affinity, in the very same sense that we have a me- 
chanical tfheory of heal. 
“As an example to illustrate the first thesis, the reader 
needs to be reminded only of the chemical relation that 
exists between acids and bases : sulphuric acid against 
alkalies, such as caustic potassa or quick-lime. The pro- 
cess of saturation is accompanied by the liberation of con- 
siderable heat ; the products have a very high melting 
point and are chemically indifferent. 
“To illustrate the second thesis, a good example is the 
formation of carbon sulphide. As is well known, this pro- 
cess requires a considerable expenditure of heat, and the 
product thus obtained is volatile at a temperature far below 
the degree of temperature required for its formation. This 
heat, taken up by the carbon sulphide, is contained in it as 
chemical affinity, and is evident by its low specific gravity, 
its low boiling point, its proneness to decomposition, and 
the increased development of heat during the combustion of 
its elements.” 
The great principle underlying all this reasoning is that 
of “ the conservation of energy and the correlation of forces," 
of which, as we have shown, he was the first exponent.! 
* Archiv der Pharmacie ; Vol. 216, 1880. 
t See the paper published in No. 17 of “ Science.” 
