March 2, 1871] 
with in almost every family where they are cherished and 
loved. Some of the numerous illustrations are by Harrison 
Weir, others after Landseer; they are very attractive, 
though few of them are new. 
Both volumes are got up in that attractive style with 
which we are familiar from Mr. Partridge, and make very 
pretty books for presents to young people. 
Class Book of Inorganic Chemistry. By D. Morris, B.A. 
(London: George Philip and Son, 1870.) 
THIS text-book is specially designed for pupils preparing 
for the Oxford and Cambridge Middle-class examinations 
and for the matriculation examination of the University 
of London. How far it will answer its purpose will be 
seen best by the number of students who use it as their 
guide and succeed in passing such examinations. Written 
for a special purpose, it is exempt from the criticism to which 
a general text-book would be subject, and we shall there- 
fore amply point out how the book fails, in our opinion, to 
accomplish its purpose. 
Nothing is more important for a student than that 
the definitions he learns should be clear and precise ; 
yet those in this work are almost uniformly bad. 
Absolute weight, for example, is defined as “the 
amount of weighable matter in a body,” which might 
have been simplified by saying, “weight is weight.” 
Again, solution is defined as the “ perfect union of a solid 
with a fluid,” thus making the hydration of caustic lime 
an example of solution, and excluding the solution of 
gases in liquids. 
It would be unjust to the author to state which nomen- 
clature he has adopted ; he has shown his impartiality by 
adopting nearly every one proposed ; “ potass,” “potash,” 
“potassium hydrate,” and “caustic potash,” are used in- 
differently ; “sulphate of potassium” and “ dipotassic sul- 
phate ” are used as synonyms for the substance having the 
formula HKSO,. But the most objectionable feature in 
the book is a certain looseness of expression, leading to 
positive errors, which is certain to perplex the student. 
Examples of this abound; we need only quote a few. 
The solution of ammonia in water, we are told, “has all 
the properties of the gas;” again, “strontia forms with 
water a hydrate which has all the properties of baryta 
water,” and “lime-water has all the properties of solu- 
tions of potash and soda.” 
Several statements occur in the book which are so ab- 
surd that they can only be traced to careless revision ; but 
although they will scarcely mislead the merest tyro in the 
study of chemistry, they are none the less objectionable. 
We thought it very absurd to read that “combustion in 
air and in oxygen is exactly the same thing,” and we 
thought it more absurd to read that “ammonium and 
sodium are distinguished by the smell of ammonia on the 
-addition of caustic potash,” but we only arrived at the 
climax of absurdity when we read that “ nitrogen increases 
“the volume of the atmosphere ; and in this way provision 
is made for winds and other things useful for man’s well 
being.” Surely after this the text-books will cease to tell 
us that nitrogen is a very inactive substance. 
It is only fair to say that the latter portions of the book 
are tolerably free from such errors as we have noticed ; 
and the chapters on the heavier metals are the best in the 
book. Questions selected from the examination papers 
of Oxford, Cambridge, and London Universities, and the 
Science and Art Department, are given in several places, 
and the book concludes with tables for the analysis of 
simple salts. It is doubtful if a work of the size and 
scope of the one before us was at all needed, when 
there are so many excellent small manuals ; but if such 
a one is really required, it must be much more carefully 
compiled and edited before it can be either useful or 
instructive, 
‘ F. J. 
NATURE 

345 

LETTERS TO THE EDITOR 
[Zhe Editor does not hold himself responsible for opinions expressed 
by his Correspondents. No notice is taken of anonymous 
communications. ] 
Measurement of Mass and Force 
““W. M. W.” seems to regard units of mass and force as things 
which are to be talked and philosophised about, rather than to 
be actually used for the measurement of concrete qualities. Itis 
true that the force of gravity on a standard pound at a specified 
locality is a definite unit of force ; but instead of specifying a 
locality to which reference is by common consent to be made 
(and without which consent we shall have as many different units 
as there are localities), he says, ‘‘The assumption of a hypo- 
thetical force of gravity not dependent on latitude seems to 
stand on the same footing as the employment of a mean solar day.” 
The average length of the apparent solar day is the same at 
all places on the earth, and is called a mean solar day. I am 
curious to see ““W. M. W.’s” definition of a hypothetical force of 
gravity not dependent on latitude. It appears to me to be such 
stuff as dreams are made of. 
As your readers are probably nearly tired of this discussion, I 
propose to conclude it on my side by the following summary of 
the whole question :— 
Mass and quantity of matter are generally identified, and were 
identified by Newton in the opening paragraph of the Principia ; 
but Iam disposed to agree with ‘*W. M. W.” so far as to allow 
that the comparison of quantity of dissimilar kinds of matter is 
to a certain extent conventional, though there is, perhaps, no 
other convention which has so strong a basis of reason. If 
it is the case that all kinds of matter consist of one elemen- 
tary material differently arranged, this convention must of 
necessity be no longer a convention, but a simple statement 
of fact, and portions of dissimilar matter which have equal 
masses, whether as measured by the inertia test or the gravitation 
test, consist of equal quantities of the elementary material. The 
rigorous agreement which is known to hold between these two 
tests, as applied to the comparison of different kinds of matter, 
may even be quoted as an argument for the existence of such a 
common material. 
Whatever we may think of this convention or assumption 
(call it which we will), the phrase a pound of matter is legitimate 
and inyolves no assumption. A pound of sugar is a pound of 
matter, and a pound of lead is a pound of matter, independent 
of all hypothesis. 
All authorities are agreed that, in treating kinetical problems, 
it is expedient to express masses and forces in such units as to 
satisfy the relation, 
Force = Mass x Acceleration, 
which, for a body, falling freely in vacuo, becomes 
Gravitating force = Mass x g. 
Now comes the point of divergence, the main question being 
this : shall we (I.) measure force by reference to a standard of 
mass, or shall we (II.) measure mass by reference to a certain 
(or uncertain) standard of force. I showed in my last letter 
(p. 228) that the pound is practically, in going from place to 
place, a standard of mass, not of force, and that masses in 
different localities can be more directly compared than forces. 
It is surely reasonable to measure force by reference to a univer- 
sally accessible standard of mass, rather than to measure mass by 
reference to a standard of force which, on the earth generally, is 
difficult of identification, I therefore uphold the following 
system :— 
I. Take a pound of matteras the unit of mass; from which 
it follows that the gravitating force of a pound of matter has the 
“ : ae : 
numerical value g, and the unit of force is — of the force with 
which a pound of matter gravitates. 
This system was first proposed by Gauss, for the comparison 
of magnetic forces in different parts of the earth, It is the 
system which has been always taught by Sir William Thomson, 
and is adopted in Thomson and Tait’s Natural Philosophy, 
Tait and Steele’s Dynamics (except the first edition), the Reports 
of the Brit. Assoc. Committee on Electrical Standards (see 
especially the Report for 1863), Balfour Stewart's Elementary 
Physics, and the later editions of Atkinson's Ganot. 
If we adopt the other alternative, we are driven to choose 
between certain subordinate alternatives, of which, with ‘‘ W 
_M. W.’s” help, I may enumerate three, 
