a eS 
Fan. 5, 1871] 
NATURE 
187 

equivalents for such groups as the hawks and doves within its | 
limits. Whereas it seems to me that the truer parallel is between 
the whole class Insecta and Birds, and that the equivalent 
groups for hawks, doves, &c., are to be looked for, not in one 
of the sections, but in the whole of the class. 
hawks and doves in the Lepidoptera. I find nothing but doves. 
If you want hawks you must go to the dragon-flies, which are 
their equivalent ; and, of course, if we are only dealing with 
doyes, there is nothing in the known phenomena of hybridisation 
opposed to such a cross having taken place. 
It is impossible in the brief space that you would allow me, 
even to glance at the many arguments that I could adduce to 
show that this is the true position of the Zefzdoptera. I hope to 
do so elsewhere. But I would only remind entomologists, 
especially lepidopterists, of the trifling characters on which their 
genera have been established, and how difficult it has been to 
find any generic characters at all. This is frankly acknowledged 
as the great difficulty attending the study of Zefidoptera, conse- 
quently characters which would never for amoment be looked on 
as generic in any other group of animals, are there allowed that 
value. If any specialist in another group objects, what is the 
answer? ‘‘ We have no better characters, and we must do the 
best we can with the slight ones we possess.” Quite right, in a 
systematic point of view. If the species of doves came to be 
reckoned by thousands, the ornithologist would just have to do 
the same thing ; but that would not alter the position of doves 
in the animal kingdom—they would still bear the same relation 
that they do now to hawks, and be equally open to hybri- | 
disation among themselves, indeed, more so; for such great 
numbers of one type would be a presumption in favour of 
every mode by which species could be increased having been 
resorted to ; and this by the way is an additional indirect argu- 
ment in favour of hybridisation sometimes taking place among 
Lepidoptera, 
Of course, I do not mean to say that there is nothing more 
than specific distinction between the Danaids and Nymphalids. 
I recognise them as good genera, but only as genera sufficiently 
nearly akin to allow of hybridisation taking place between them— | 
and ecce signum—the mimics in question partaking of the 
characters of each in all respects as other hybrids do, 
ANDREW MURRAY 
67, Bedford Gardens, Kensington, Dec.30, 1870 

Measurement of Mass 
THE favourite definition of mass in the text-books seems to be 
that the mass of a body is the geantity of matter it contains. If 
we had to do with but one kind of matter this would be in- 
telligible, but I am at a loss to know what is meant when it is 
said that a piece of cork contains as much matter as a piece of 
lead. The only satisfactory method of explaining what is meant 
by the mass of a body, is to define it as a constant belonging to 
the body, which expresses the proportion between the force 
(measured statically) acting upon it and the acceleration pro- 
duced ; that every body has such a constant is the result of experi- 
ment. The mass of a body has no necessary connection with its 
weight. We employ weight to measure mass simply because 
gravity is a convenient constant force. If then we adopt a pound 
as our unit of weight, and use g to denote the force of gravity in 
reference toa foot and a second as the units of length and time, 
our unit of mass becomes the mass of g pounds, and this is not 
variable, although the unit of weight employed 1s variable ; since 
if a true pound, as determined at London, were carried to the 
North Pole, it would weigh more than a pound, precisely in the 
proportion in which gravity at the Pole is greater than gravity at 
London. 
THe REVIEWER OF EVERETT’S ‘‘ DESCHANEL” 


PHOTOGRAPHIC PROCESSES OF THE 
PRESENT DAY 
ape last two or three years will certainly mark an era 
in Photography, for not only have several novel and 
important printing methods been discovered during that 
_period, but other processes of less recent origin have of 
late been so elaborated and improved as to have become 
He looks for both | 


at the present moment practical and easy of manipulation. 
All of these are, without exception, based upon the action 
of light upon the bichromates of potash and ammonia ; 
In no single case is the use of a silver salt involved—the 
agent employed for securing the photographic image in 
ordinary paper printing—and this is, in truth, a point 
whose value cannot be too greatly insisted on; for the 
silver print, be it washed and freed as thoroughly as 
possible from any deleterious bodies, will always suffer, 
more or less, from attacks of an impure atmosphere, the 
delicate metallic film of which the image consists being 
peculiarly liable to change, from the sulphur compounds 
and other impurities not unfrequently contained in the 
air we breathe. And even those silver pictures which do 
not at first show actual traces of fading or discoloration, 
will very soon be found, on careful examination, to have 
parted with some of their original brilliancy, and to lack 
the pristine freshness which always characterises newly- 
produced albumenised prints. 
It is a great step onwards, then, to have at our disposal 
practical processes in which the employment of silver may 
be altogether dispensed with, by the substitution of another 
material of a more permanent character, either in the 
form of a chromium compound, or, what is better still, in 
the shape of gelatinous or greasy ink; and so clear and 
promising does the photographic horizon appear just now 
in this direction, as to leave little ground for doubting 
that before long the practice of printing, in silver will be 
generally abandoned. 
All recent printing processes rest,as we have before 
said, on the action of light upon the bichromates, and 
here we would parenthetically refer to a simple and 
| familiar experiment which will help very materially to 
simplify our subsequent remarks. The well-known plan 
pursued by school-boys for printing fern-leaves and other 
objects by the aid of the sun, will readily be called to 
mind by many of us, and this simple manipulation it is 
that forms the groundwork of the whole series of in- 
ventions before us. A sheet of ordinary paper, which has 
of course been sized, or, in other words, received a thin 
coating of gelatine, is rubbed over with a solution of bichro- 
mate of potash ; the latter, as we know, when mixed with 
any organic body renders the same sensitive to light, and 
the sizing or gelatine upon the paper becomes in this way 
endowed with excitable properties. Having been dried 
in the dark, our sheet of paper is next placed in the sun with 
the fern-leaf, or other object to be copied, pressed down 
uponit, and the light acting upon all such portions of the 
sheet as are not covered up, browns the gelatine there and 
renders it insoluble ; the sizing underneath the leaf, and 
screened therefore from the light, escapes this reaction 
and remains soluble, and this, on the printing being com- 
pleted and the paper washed in water, is at once dissolved 
away, there remaining a white image of the leaf upon a 
brown ground composed of bichromated gelatine rendered 
insoluble by the sun’s rays. This experiment may be 
regarded as the key to the whole question of photo- 
graphic printing, and by bearing it in mind the reader 
will have no difficulty in at once comprehending the 
various inventions of the kind just now being made 
public. 
The first method claiming our attention is the so-called 
carbon process. Photographic printing of this nature in 
one form or another has been carried on probably for up- 
wards of fifteen years ; but in its experimental stage the 
mediocre character of the results furnished by it were such 
as to deprive the system of any material support from 
photographers, and until, in fact, Mr. J. W. Swan, of New- 
castle, made known his method, no easy or reliable modus 
operandi can be said to have existed. The plan followed 
by Mr. Swan was to prepare a warm solution of gelatine 
and bichromate of potash mixed with some finely divided 
pigments, such, for instance, as Indian Ink, and apply 
this mixture in the form of a coating to a sheet of paper, 
\ 
