Cfje $o|)ular Science iSetus 



AND 



BOSTON JOURNAL OF CHEMISTRY. 



Volume XXII. 



BOSTON, AUGUST, 1888. 



Number 8. 



CONTENTS. 



Familiar Science. — The Photography of Moving Projec- 

 tiles 113 



Fire-Drawings 113 



Kent's Cavern ; its Wonderful Story of Prehistoric Man.. 114 



Paris Letter 114 



Trap-Door Spiders and their Nests llo 



The Grand Magnet at Willett's Point 116 



An Unpolishable Diamond 116 



Scientific Brevities 116 



Practical Chemistry and the Arts. — Glass-Making . . 117 



Magic Mirrors 117 



Metallic Alloys 117 



Faclsabout K,illroads. 118 



Procuring Chemicals under Difficulties 118 



Workshop Hints 118 



Home, Farm, and Garden. — A Tent of Living Vines 119 



Selected Recipes 119 



How to Blanch Celery 119 



The Luxury of a Rose- Jar 120 



How to Cleanse Chamois 120 



To Keep a Room Cool 120 



Household Hints .• 120 



Gleanings 120 



Editorial. — The Future of Chemistry 121 



Precipitation 122 



A Fossil Human Footprint 123 



The People and the Schools 123 



Meteorology for June, 1888 123 



Astronomical Phenomena for August, 1888 124 



Literary Notes 124 



Medicine and Pharmacy. — Pneumatic Massage 125 



Antiseptic Surgery , 125 



Monthly Summary of Medical Progress 126 



A Philosopher's Prescriptions 126 



The Anwsthetic Revelation 127 



Salicylic Acid in France 127 



The Feeding of Children 127 



Anti -Vaccination Theorists 128 



Medical Memoranda 128 



Pdblishers' Column 112 



familiar Sxiencr. 



THE PHOTOGRAPHY OF MOVING 

 PROJECTILES. 



Some time ago the fact was noted in this 

 paper that a European scientist had succeeded 

 in obtaining a direct photograph of a bullet in 

 motion, just after being discharged from the 

 gun. We reproduce from La Nature some 

 engravings made from photographs obtained by 

 Professor Salcher of Fiume, Austria. 



It is evident that the air in front of a body 

 moving as rapidly as a bullet must be more or 

 less condensed ; and, on the contrar}', the air 

 behind it will be somewhat rarified. As the 

 refractive power of air varies with its density-. 



these effects ought to be shown very plainly in 

 a photograph, where the light affecting the 

 plate passes through these strata of air of 

 varying density ; and, on inspection of the en- 

 gravings, we find the head, or wave of con- 



densed air, in front of the projectile very 

 clearly marked. We also see in the rear of 

 Fig. 3 the spiral eddies or whirlpools formed 

 in the rarified air behind it. 



When it is remembered that the velocit}- of 

 these bullets at the moment of their being pho- 

 tographed was from thirteen hundred to six- 

 teen hundred feet per second, it would seem 

 almost impossible to impress their image upon 

 the sensitive plate. The diagram (P'ig. 1) 

 shows the principle upon which the feat was 

 accomplished. The camera, *4, is placed at 

 the proper distance ; and in front of it is placed 

 a screen, E, which cuts off all rays of light, 

 except those proceeding from the projectile. 



Fl9. 2. 



Fie. 3. 



Fio. 4. 



Fig. 5. 



At X is a lens which concentrates the image 

 upon the camera. 5 is a Leyden jar, with 

 wires attached connecting the outer and inner 

 surfaces. The circuit is interrupted at H and 

 /. The projectile p is fired in such a direction 

 that it passes between the wires at /, closing 

 the circuit, and causing an electric spark to 

 appear both at / and H. The former spark 

 illuminates the projectile, and the light from 

 the other passes through the air surrounding 

 it. It will be seen that the action is entirely 

 automatic ; the flying bullet illuminating itself 

 at just the right moment, and for a small frac- 

 tion of a second only. The whole action is 

 practicallj- Instantaneous, and shows the won- 

 derful sensitiveness to which the modern dry 

 plates have been brought. The time of expos- 

 ure of the plate must in these cases have been 

 less than one hundred thousandth of a second. 

 In Figs. 2 and 3 the projectile was moving 

 at the rate of thirteen hundred feet a second. 

 These show the layer of condensed air in front 

 of it particularly well. Fig. 5 shows, in addi- 

 tion, the electrodes or wires between which the 



bullet passed, while Fig. 4 shows the eddies 

 and little aerial whirlpools behind it. The 

 velocity of the projectile in the last two figures 

 was sixteen hundred feet a second. 



The original photographs were necessarily 

 very small, only about three-sixteenths of an 

 inch in diameter ; but they were carefully 

 copied and enlarged by the engraver, without 

 exaggerating the details in the slightest 

 degree. 



FIRE-DRAWINGS. 



A VERY amusing scientific experiment may- 

 be performed by taking a saturated solution of 

 nitrate of potash (saltpetre), and, with a quill- 

 pen or fine brush, drawing any picture, design, 

 or words upon a piece of white absorbent 

 paper. The lines should be kept away from 

 each other ; and the entire subject coarsely 

 drawn in outline, like the elephant in the illus- 

 tration. When dry, the lines will be nearly 

 invisible ; but if one of them be touched with 

 the glowing end of an extinguished match, a 

 spark of fire will run through the paper, fol- 

 lowing the lines already traced, and cutting 

 out the design as if with an invisible knife. 



This experiment is explained by the chemi- 

 cal constitution of the saltpetre. This salt 

 contains a large amount of oxygen, so loosely 

 combined that it readily leaves the nitrogen 

 and potash, and unites with the carbon of the 

 paper, when heated to the point of ignition. 

 The heat developed by the combustion is not 

 sufficient to ignite the paper, except where it 

 has been saturated with the oxygen-giving 

 salt ; and so the spark of fire, which is really 

 only an indication of a violent chemical re- 

 action, follows the lines previously traced. If 

 an actual flame was brought in contact with 

 the paper, of course the whole would be con- 



sumed ; but the heat of the glowing charcoal is 

 just sufficient to start the combustion, by the 

 aid of the oxj'gen in the saltpetre. 



The explosion of gunpowder is due to the 

 same cause. It is composed of charcoal (car- 

 bon), sulphur, and saltpetre, intimately mixed. 



