FEHK^Al<^, 1012. 



kn()\\ij:i)(",i:. 



SI 



birds. He found also that the birds cannot soar if the sky is 

 densely overcast : either direct sunshine or at least good 

 sunlight are necessary. His conclusion is that the sinilight 

 stores up in the air some form of potential energy which by 

 the passage of the bird's wings becomes liberated as kinetic 

 energy and gives the 



sustaining power. Minute '' 



explosions or molecular 

 bombardment arc terms 

 which express the idea 

 of the Hbcration of the 

 energy. To this supposed 

 allotropic form of the 

 atmosphere Dr. Hankin 

 gives the name of 

 " ergaer." People who 

 have not followed his 

 record of observations 

 and careful scientific 

 reasoning may be inclined 

 to smile, but he makes 

 a good case, and points 

 to lines of original re- 

 search which his idea — 

 really a working theory — 

 opens out. Mr. Hankin 

 lectures to the Aero- 

 nautical Society of 

 Great Britain on January 

 22nd, and he may then 

 p;)ssibly say something 



more on his " ergaer " theorv. 



PIIOTOliR.MMIW 

 By Edgar Semoi;. 

 I-IKIHI-R NOTES ON PRINTIN(; WITH IRON 

 S-ALTS. — -In "'Notes" of last issue the method of iron 

 printing described gives i^ 



white lines on a blue 

 ground, "unless a nega- 

 tive be employed," in 

 other words, the prints 

 themselves are negative 

 ones. This is not always 

 desirable. As to whether 

 the prints produced are 

 negative or positive de- 

 pends upon whether the 

 agent used to develop 

 the blue compound re- 

 acts with the altered or 

 unaltered salt. In the 

 case already considered, 

 theblue compound results 

 from the reaction of 

 the potassium ferri- 

 cyanide (red prussiate 

 of potash I, with the 

 ferrous salt produced 

 by the action of light. 

 If, however, potassium In.r 



ferrocyanide (yellow 



prussiate of potash* be used instead, the blue compound results, 

 from its action upon the unaltered (ferrici salt forming the com- 

 pound known as Prussian blue Fci (Fe Cy,;l:i. Advantage is 

 taken of this for obtaining positives from positives direct. In 

 order to carry this out in practice the following solutions are 

 made, and the mixture applied to well-sized paper in a 

 subdued light : — I. 



Ferric chloride ... ... ... .U grains. 



Tartaric acid ... ... ^ ... ... 17 



Sodium chloride ... ... ... 1.! 



Water 1 "^. 



Gnm-aiab 

 Water . . . 



107 grains. 

 1 oz. 



These two solutions are mi.xed and then applied to the 

 paper by moans of a brush in as even a manner as possible, 

 and dried ijuickly in front of a fire in order that they shall not 

 sink into the pores. The printing may be done in sunlight in 

 a few minutes, or in iliiTiised daylight for a correspondingly 



longer time, but a little 

 ^ experience soon enables 



one to judge by appear- 

 ance when the printing 

 has been carried far 

 enough. To develop the 

 print a strong solution of 

 potassium ferrocyanide 

 is used, floating the ex- 

 posed surface upon the 

 solution, care being taken 

 3 not to wet the back. 



i When fully developed, 



J float on clean water for a 



J. few minutes and then 



t immerse in a ten per 



f cent, solution of hydro- 



t chloric acid for a short 



\ time and then thoroughly 



iwash. If the ground is 

 stained blue the print 

 has been under-exposed 

 probably, and with care 

 the white portions should 

 be quite free from any 

 blue colouration, the gum being used in order to insure this. 

 With over-exposure, however, it fre<iuently happens that the 

 ground is stained a light blue tint, due to the polassiuui 

 ferrocyanide giving with ferrous salts a bluish-white precipitate 

 of potassium ferrous ferrocyanide of the following compo-sition 

 IK.> Fe" Fe Cyi;). In some instances, it has been found 

 advantageous to print through the paper, in which case the 

 object being copied must 

 be placed in its proper 

 j , jj aspect with regard to left 



I and right, otherwise the 



* inint will be reversed. 



J si'Ri:ading of 



I r H I-; I M A GEO F 



I FIN1-: LINES. — Any- 



1. one who has done much 



* work in the photograph- 

 f ing of fine lines, or struc- 

 '1 tures of that kind, nmst 

 i have noticed the general 

 ■ thickening - up which 

 it takes place, rendering 

 $ the image coarser and 

 i unlike the original in 



.ippearance. This is well 

 ^ shown in the accompany- 



i" ing illustrations, which 



» are photo - micrographs 



enlarged sixty diameters. 

 :i; ,s.;. In F'igure 82 a we have 



the enlarged image of 

 two fine lines separated by a distance of js millimetre 

 (i;io in.), and in Figure 82 b an image of the same 

 two lines from a negative that had received double the 

 exposure of a. It will be seen that not only has the space 

 between almost entirely disappeared by the prolonged 

 exposure, but that the spreading has extended to the outer 

 edges as well, to such an extent that the total increase in the 

 width of /; as compared with a is nearly three millimetres 

 in the enlarged image. 



In two other photographs on the same plate, in which the 

 exposures had been further increased by doubling each time, 

 the separation between the lines had become quite clo.scd up 

 and still further spreading occurred, so that two fine lines 

 had become one thick one. It may be argued that this case 



