July 20, 1876] 



NATURE 



255 



alluvium near Oxford, and from Germany 3. Skull of Sus 

 andamanensis, forwarded him by J. Wood Mason. 4. Skull of 

 Sus cristatus, lent by Sir Walter Elliott, K.C.S.T. 5, Skull of 

 Sus barbahis wrongly named 5. verrucosus, and needlessly 

 Euhys barbatus in some mammalogical catalogues. 



Froni these and other data the author bases the subjoined 

 conclusions : — 



1. The domesticated pig of Pre-Roman times, as exemplified 

 at least by the specimens from the interment referred to, appear 

 to resemble Sus scrofa, van ferus, rather than S. cristatus, or the 

 domestic variety, S. indicus. 



2. On the other hand, .5. cristatus, the Indian wild bog, 

 appears to him, whilst being readily and always distinguishable 

 from S. scrofa, van Jerus, to differ from it, mainly by the 

 retention permanently of certain structural conformations which 

 were only temporarily represented in the European wild species. 

 The third molars of the male, S. cristatus, varied, however, con- 

 comitantly with its canines, and showed a much larger develop- 

 ment of their posterior lobe, than either S. scro/a, van /ems, or 

 the females of their own species. The rearmost lobe, however, 

 of the posterior molar, varies a good deal in S. scro/a, van ferus, 

 irrespective of sex. 



3. Bearing in mind the elasticity of the swine type and the 

 power for changing which their domestication has shown to 

 possess. Dr. Rolleston has less difficulty in conceiving that the 

 so-called .5". indicus was really a modified S. cristatus, than that 

 ic had been evolved fiom any Sus, such as S. Icucomystax, from 

 countiies farther away from Europe than India. S. cristatush&d 

 the malar border of the lachrymal alwajs marked by the rela- 

 tive shortness insisted on by Nathusius. It had not the lelatively 

 wider palate ; but upon this point too much weight had been 

 laid. 



4. A skull of a Vv^ild sow, from the alluvium, later in date 

 than the "river gravels," near Oxford, combmed the short 

 lachrymal characteristic of young pigs and of S. cristatus, 

 with the worn down teeth, elongated lacial skeleton, and dis- 

 proportionally small size of an old wild sow, S. scro/a, \a.r./erus. 

 Such a combmation of characteristics tended to suggest careiul- 

 ness as to accepting the Torf-Schwein S. scrofa, yax. palustris, of 

 Rlitiinejer, as a distinct species, or taking even such a point as 

 the shortness of the lachrymal as constituting a specific differ- 

 ence. 



5. The simplicity of the third molars in the very large skull 

 of S. barbatus appear to be of greater value, as the rugose 

 condition might have been expected to be forthcoming in so 

 large, so well armed, and so well fed a Sus as this from Borneo. 



6. The true S. verrucosus differs from S. barbatus in having 

 the lachrymal's malar edges long, relatively to its orbital, as 

 well as in the peculiarities v/hich its specific name implies. 

 These peculiarities were reproduced in the old Irish " Greyhound 

 P'g" figured by Richardson "Domestic Pigs," p. 49, Ed. 

 Warne. 



7. The often-quoted paper by Dr. Gordon, Medical Times 

 and Gazette, May 2, 1857, p. 429, led us to suppose that Taenia 

 solium of man, infested the domestic pig of India, as it does 

 those of other parts of the world. The facility with which 

 the pig lends itself to domestication enables us to under- 

 stand how the many-sided commensalism which now exists 

 between man and that animal may have set up in very early 

 times. Indeed the particular results of their commensalism 

 which their solidarity, as regards the alterations of the genera- 

 tions of Taenia solium represents, suggests that their co-existence 

 in lime must have been more extensive than even the co-exist- 

 ence in space ascribed to them, not quite correctly, by Gibbon 

 ("Decline and Fall," chap. ix. note 9, p. 392, Smith's edition). 



PHOTOGRAPHIC PROCESSES 1 

 II. 



W 



^E next pass on to other applications of the dichromates for 

 the production of prints, and the first I shall demonstrate is 

 that known as carbon printing, but which is perhaps more cor- 

 rectly termed the autotype process. It is dependent on the oxida- 

 tion of gelatine, one of the substances which you may have already 

 guessed would be capable of being acted upon by the dichromates. 

 ii, then, we have a film of this gelatine impregnated with potas- 

 sium dichromate, and after drying it be exposed to light, it will 

 be found that all the portions acted upon will become insoluble 



* Lecture by Capt. Abney, R.E., F.R.S., at the Loan Collection, South 

 Kensington. Continued from p. 241. 



in hot water ; that is, supposing the duration of the expo:.ure be 

 of sufficient duration, and if the light be sufficiently intense. 

 Imagine now that beneath a negative of delicate gradations of 

 light and shade we place a film of sensitive gelatine, supported 

 for convenience' sake on paper, and allow sunlight to act upon it:. 

 After a time, in what condition will the gelatine be? It will be 

 partially insoluble, more particularly on the surface next the 

 negative, and the lights and shades will be represented by dif- 

 ferent depths of insoluble matter, according to the intensity of 

 light penetrating through the various parts of the negative. I 

 must here pause, and try and explain why this is. Ac fiist sight 

 it might seem that the whole of the tfUchiess of t lie film ought to 

 possess different ratios of solubility. Tnis is not true, however • 

 the solubility is affected to different depths. Tnat coloured 

 component of white light which is principally effective in 

 producing the chemical change is blue, and which conse- 

 quently finds a difficulty in piercing through the orange- 

 cqloured dichromate. The amplitude or heigut of the blue 

 wave is continually diminished, till finally it is almosc 

 extinguished. Now the intenser the wliite light the greater 

 will be the original amplitude of this wave, and it is at once 

 apparent that the limit of amplitude, which is effective to cause 

 the chemical change, will be reached at a gteater depth by those 

 rays of light which were originally the brightest. A little 

 reflection, then, will show you that the soluble part of the 

 gelatine will principally be next the paper, and on immersion in 

 hot water the viscous unaltered gelatine would remain imbedded 

 between it and the outer insoluble surface. Though several 

 ingenious methods have been tried to render the support on 

 which the gelatine rested sufficiently porous to allow the occluded 

 parts to be washed away, yet, so far, no attempt has been com- 

 pletely successful. To get over the difficulty the principle has 

 been adopted of transferring the gelatine film to a temporary 

 support, the outside surface being caused to adhere to it. Evi- 

 dently, by this means, the soluble gelatine can be washed away 

 when the paper is peeled off, and a raised image insoluble in 

 water would remain, which eventually may be transferred to its 

 final support. The temporary supports, usually employed are 

 metal plates, glass, paper coated with an insoluble com- 

 pound, &c. A picture in gelatine alone, however, would 

 be, comparatively speaking, of little value, as it is almost colour- 

 less ; but if pigments be mixed with it the objection disappears.. 

 In the autotype process the gelatine is mixed with colouring 

 matter and a coating is given to a piece of papen When dried 

 the gelatine can be rendered sensitive by floating its surface on a 

 solution of potassium dichromate, and after again drying is ready 

 for printing. Such a piece of prepared paper, or carbon tissue 

 as ic is technically called, we have here. It has already been 

 exposed beneath a negative, but no trace of any image is appa- 

 rent, as the dark colour of the pigment masks it entirely. 

 In order to judge of the amount of hght received during exposure 

 resort then is had to what are called actmometers. The detail 

 of the instruments I will not enter into ; suffice it to say it is usual 

 to judge the depth of printing by the colour given to silver 

 chloride. Placing then the exposed tissue, gelatine side down- 

 wards, beneath water in which a zinc plate has already been 

 immersed, and bringing the surfaces of the two together, they are 

 withdrawn from the water with a film of moisture between. 

 You will notice that I left the print in the dish but a very short 

 time, for a reason which you will presently understand. By 

 passing this "squeegee" (which is a bar of wood from which a 

 thick strip of india-rubber projects) over the back of the paper I 

 drive out all the water from between the surfaces, and you see how 

 the gelatine film clings to the zinc. And why is this ? You will 

 find that it is not naturally adhesive, the light has changed the 

 ■quality of the gelatine in this respect, then why does it hold so- 

 tight to the metal plate ? Simply owing to the moisture left 

 in the paper ; the soluble gelatine soaks it up and expands. 

 It cannot well expand laterally, so it expands upwards, and a 

 partial vacuum is created between the gelatine and the plate. 

 Now you see why I left the print in the water such a short time. 

 Had 1 left it in longer the total expansion would have taken 

 place, and the necessary vacuum coiild not have been created 

 when it was pressed on to the zinc plate. 



Now that it is firmly held, I can place it in hot water and 

 remove the papen It easily peels off, and the solvent action of 

 the fluid can have fair play. As I move it up and down in the 

 trough, you can see the gelatine running over the surface. After 

 a few mmutes it is clean, and the development is finished. On 

 this plate I have another print which has already undergone 

 similar treatment, but has been allowed to dry. This piece of 



