KNo\vij;i)r,i': 



Jani'arv. 1912. 



riu' objects from which it is desired to obtain images are laid 

 upon a piece of ^'lass placed in a prinlinK frame, and the 

 paper placed with its coated surface in contact ; the whole is 

 then ex|K>sed Id HkIiI fur a time varvinR from five minutes in a 

 bri(!l)t liKht to twenty minutes or more in a dull one. How 

 ever, as the chauKe bronchi about is a visible one. the progress 

 of printing may be watched. ex.nniiiinK the print in some wcii- 

 sliaded position : especially so is this necessary with paper 

 prepared with the urcen ferric-aumionium citrate, on account 

 of its increased sensitiveness. 



If ordinary negatives are used to print from (and excellent 

 results can readily be obtained in this wayl care umst be 

 taken not to over-expose: in fact, the printing must not be 

 carried to snch a stage that the det.iil in the hghts becomes 

 visible, otherwise a general flatness of the picture will result. 

 The negatives employed must be fairly strong ones, it being 

 useless to attempt to use poor thin ones. The change in 

 colour during exposure to light is from a greenish-yellow to a 

 pale greyish-brown, a little experience soon enabling one to 

 judge when the printing has been carried far enough. On 

 removal from the printing frame development may take place 

 at once, or the paper may be placed in a book until a more 

 convenient time. Experience, however, shows that too long 

 ;i time should not be allowed to elapse between the two 

 operations. In order to develop the print it is laid face up- 

 wards in a porcelain dish, and a solution of potassium ferri- 

 cyanide (red prnssiate of potash) of about twenty grains in an 

 ounce of water poured over. Development is almost instan- 

 taneous, the print assuming a fine blue colour due to the 

 ferrous salt resulting from the action of the light reacting 

 with the ferricyanide of potassium, and producing a pre- 

 cipitate known as Turnbull's blue, having the composition 

 Fei Fe-jCyij. After development the prints are placed 

 in water containing a little sulphuric or hydrochloric 

 acid, then washed in several changes of water and 

 dried. Prints made by this process are very stable, although 

 soaking in water certainly weakens them. A variation of the 

 process may be made by applying to the exposed paper a 

 dilute and neutral solution of gold chloride, when a purple 

 image results, owing to the ferrous salt reducing the gold salt 

 to the metallic state. Sir John Herschel gave the name of 

 Chrysotype to these prints. 



A variation in the method described above by which blue 

 prints may be obtained direct without development may he 

 made by mixing together the ferricyanide of potassium and 

 the ferric salt, and applying the same to paper. The following 

 will be found a good formula : — 

 1. 



Potassium ferricyanide ... 60 grains 



Water (distilled) ... ... 1 ounce 



2. 

 Ferric ammonium-citrate... ... ... 70 grains 



Water (distilled) ... ... ... ... 1 ounce 



These two solutions should be mixed together and kept in 

 the dark. When required for use a little is taken and .spread 

 over paper, as already described, which when dry is ready for 

 use. When the .action of light has gone far enough, " which 

 is ascertained by examination," the print is removed from 

 the frame and placed in water containing a few drops of 

 sulphuric acid, and afterwards washed in several changes and 

 placed to dry. To make uranium prints the same procedure 

 as described above is employed, using a strong solution of 

 uranium nitrate of about one hundred grains in one ounce of 

 water in place of the ammonio-citratc of iron. Development 

 of the prints with potassium-ferricyauide gives brown i)rints. 

 If we mix uranium and the iron salt, and apply this to p.iper, 

 a grey print results on development; in fact, mixing of the 

 .salts in varying proportions is found to give, in many cases, 

 rather pleasing results. 



EXPOSl'RE TABLF FOR J AN'l'AKY.— Although weather 

 conditi(ms during the next month or two may not be altogether 

 conducive to outdoor photography, there may be occasions 

 when opportunities present themselves for this class of work. 

 We therefore give a table containing a few exposures that may 



be foinid useful. The calculations are made on the aclino- 

 graph for plates of speed 200 li. and D.. the object a near 

 one, and lens aperture I" If). 



I'll\ SIC.S. 



Hy Alikhd C. G. Egertox, B.Sc. 



THF.KMOSTATS. — The Faraday Society's Meetings are 

 always marked by a degree of general usefulness, interest, and 

 enthusiasm which is sometimes lacking in the proceedings of 

 older societies. On Wednesday, December 6th, among the 

 subjects for discussion was that of the construction and work- 

 ing of thermostats. Thermostats are arrangements for main- 

 taining temperatures constant over a long period of time and 

 are most necessary accessories to the study of most physico- 

 chemical processes. 



A thermostat consists generally of a bath of liquid which is 

 maintained constant in temperature by means of a heater 

 placed either inside or outside the bath. The heater, if used 

 inside the bath, consists either in a bare electric resistance 

 wire or an electric lamp ; if used externally the heating is 

 usually accomplished by means of a small gas burner. In 

 each case some form of thermo-regulator, which .switches on 

 .ind off the current or turns the gas up and down, has to be 

 employed. This thermo-regulator usually consists of a closed 

 tube of liquid with low specific heat, high expansion coefficient. 

 and small density which expands on rise of temperature : the 

 litiuid then presses on one side of the mercury contained in a 

 rtube, which on being raised in the other limb of this tube, 

 closes either an electric circuit or the end of a tube through 

 which the flow of gas to the burner is passing. In the first 

 case, the electric current actuates an electro-magnet which 

 switches out the electric heaters ; in the other case the supply 

 of gas is automatically cut down by closing the end of the 

 tube mentioned and can only issue through a small opening in 

 the side of this tube. An efficient stirring arrangement has to 

 be used in the baths, otherwise a constant temperature cannot 

 be maintained. These stirrers are constructed like a " screw " 

 and are actuated by means of an electric motor. 



Dr. Lowry has improved the thermo-regulator by making 

 the tube containing the expanding liejuid (usually toluene) as 

 large as possible, so that the maximum amount of surface is 

 exposed : there is then very little lag in the action of the 

 regulator. This is accomplished by making the tube of spiral 

 form. 



Dr. Marshall gave a number of valuable hints as to the 

 working of electrically controlled thermostats. The form of 

 electric heater he advocates, is a form of elongated electric 

 lamp which is switched in and out as before described, only 

 the thermo-regulator actuates a relay which then switches out 

 the main circuit ; the relay being actuated by a single 

 secondary cell kept permanently on the charge. A relay 

 is a sensitive electro-magnet which responds to a weak 

 current and makes an electric contact, which brings into action 

 an electric current from a stronger source. The object 

 of this is to obviate pernicious sparking on breaking the con- 

 tact at the mercury surface of the thermo-regulator. Dr. 

 Marshall also described a method of employing an ice 

 thermostat for maintaining constant temperatures from 12 C 

 to C. This type of thermostat belongs to a diflcrent class, 

 in which advantage is t.akeu of the fact that substances melt 



