SCIENTIFIC NEV\^S. 



[July 1st, i88^ 



PHOTOGRAPHIC PAPER WITH GELA- 

 TINE EMULSION. 



IT is the gelatine dry plate that has made the art of photo- 

 graphy so easy to practise, and has called so many 

 thousands of amateurs into existence. 



For many years past, practically all pictures from nega- 

 tives have been printed on sensitized albu'menized paper, 

 ;'.«., paper coated with white of egg, and made sensitive to 

 daylight by means of a solution of nitrate of silver. At 

 one time it was thought that nothing could supersede collo- 

 dion, and in the same way albumen paper is now, or rather 

 has been until recently, considered incapable of improve- 

 ment. It is claimed, however, that through the medium 

 of gelatine, a process has been discovered which bids fair 

 to take the place held so long by albumen. There are 

 already several kinds of these papers coated with sensitive 

 gelatine emulsion, but we propose, in the present instance, 

 confining our attention to that known as the "Alpha Paper," 

 made by the Britannia Works Company at Ilford. 



The old albumenized paper could be coated with albu- 

 men only a few feet at a time, but the alpha paper is 

 made in a long roll, a thousand or more yards long, and 

 two feet wide. From this roll the paper is passed on by 

 elaborately worked-out machinery into the coating depart- 

 ment, where it receives a coat of warm sensitive emulsion, 

 evenly distributed. This is cooled so as to set it, and is 

 afterwards hung up and allowed to dry. In this manner it 

 is quite easy with the proper machinery to coat half a mile 

 or more of paper at one time. When dry the paper is 

 taken down and cut up to the various sizes required. 



In order to use the alpha paper, it is placed behind thfe 

 negative, and exposed either to gaslight or daylight, or even 

 magnesium light ; but as gelatine emulsion is incomparably 

 more sensitive to light than the old sensitized albumen, a 

 shorter exposure is necessary. For negatives of ordinary 

 intensity an exposure may be required to diffused dayhght of 

 from one to twenty seconds, according to the season of the 

 year, strength of light, etc. An exposure to gaslight of from 

 half-a-minute to two minutes, according to the density of 

 the negative, will generally be found correct. Daylight 

 exposures generally give the best results ; and one plan is 

 to make the exposures in an ordinary room, about two 

 feet from a window, not directly facing the sunlight, the 

 time being about ten seconds, this depending of course on 

 the negative and intensity of light. In this manner excellent 

 results have been obtained, varying from a fine chocolate to 

 a purple brown. With shorter exposures the tones become 

 colder, while, if under exposed, black and white are the 

 only tones obtained. 



Some care is necessary in development; the print should 

 retain a warmish tone at the time of washing off the 

 developer, for if the operation be carried too far it will not 

 be susceptible to proper toning. This, after treatment 

 with the alum bath for the purpose of hardening, is the 

 next operation. 



If the print has been properly exposed and developed 

 the toning bath adds a permanent warm tone. If exposure 

 has been too long and development too short it becomes 

 with increased toning more purple and even quite pink. 

 The best tint, in our opinion — though taste varies in this 

 respect — lies between the extremes, say a rich sepia or 

 chocolate brown. This with a very little practice is 

 easily attainable. The toning can be done by gaslight, or 

 in subdued daylight, as in the case of ordinary albumen 

 prints. With the above toning bath about three or four 

 minutes will be found quite sufficient time to get excellent 

 results. If the bath be diluted, of course longer toning 

 becomes necessary, and this may often be desirable. After 



the toning comes the ordinary fixing bath of hyposulphite 

 of soda. This latter behaves -(vith the alpha paper in a 

 somewhat peculiar way, differing from the ordinary 

 albumen paper in that, with a longer soaking, the " hypo " 

 acts as a second toning bath, and so much is this the case 

 that if prints be insufficiently toned it helps matters con- 

 siderably to allow them to remain for a longer period (say 

 15-20 minutes) than that necessary for the work of fixing, 

 which is about four or five minutes. 



The permanence of photographs on this paper is a point on 

 which much stress is laid by its advocates. With gelatine 

 silver paper there is no free silver, the whole of the metal 

 being combined with a haloid body, and all the soluble 

 salts washed out previous to coating. The paper will 

 keep good for years before being used. 



.^^ 



A QUESTION IN PHYSICS. 



OME time since the following amusing problem was 

 *— ' propounded by La Nature, and more recently has been 

 reproduced by the Journal de Physique Elemcntaire : When 

 boiling coffee is'poured into a cup, the drinker must wait till 

 it cools. When sugar is added, it will of course slightly 

 lower the temperature, but without taking into account the 

 loss of heat by conduction, or the increase of volume due to 

 the sugar, the question is whether it will be best to put 

 in the sugar at once, or some time afterwards, in order that 

 the coffee may cool as rapidly as possible ? 



The coffee will cool chiefly by radiation, and its rate of 

 cooling will depend on the difference between its tempera- 

 ture and that of the bodies surrounding it. In other words, 

 the hotter the coffee is, compared with the air, the more 

 rapid will be its fall in temperature. This may be proved 

 as follows : — 



1. Let T be the initial temperature of the coffee. Then 

 if the sugar be put in at once, the temperature will be T - 1. 

 After this, radiation will take place according to Newton's 

 law, until the temperature has fallen to 0, when the coffee 

 is fit to drink. During a length of time. A, the temperature 

 has fallen T -t-9 degrees. 



2. On the other hand, suppose the coffee to cool down by 

 itself to + i, and then let the sugar be put in. It will of 

 course bring down the temperature to 0, and make the 

 coffee fit to drink. Radiation in this case has again brought 

 down the temperature the same number of degrees T - t - 0, 

 as in the first case, but the time in the second is shorter, 

 and may be represented as B < A, for the temperature was 

 higher both at the beginning and end of the period of radia- 

 tion, and therefore, according to Newton's law, the rapidity 

 of cooling was greater. 



From this it follows that the cooling of the coffee will be 

 more rapid if the sugar is added when the coffee is nearly 

 ready to drink. 



The Resistance of Snow to a Bullet. — We learn from 

 the Scientific American, that some interesting experiments 

 were recently made at Ottawa, on the resistance offered by 

 a bank of snow to the passage of a rifle bullet. It was 

 found that Martini bullets, fired into a bank of well packed 

 snow, were completely spent after traversing a distance of 

 not more than four feet. In hard packed snow, mixed with 

 ice, but not hard enough to prevent digging into it with an 

 iron shovel. Snider bullets did not penetrate more than 

 about four feet ; in perfectly dry snow, packed by natural 

 drift, but capable of being easily crushed in the hand, a 

 bullet penetrated about four feet, and in loose drifted snow 

 less than seven feet, though fired from points only twenty 

 or thirty yards distant. 



