126 PROTECTING ACTION OF A RING. 



a receiver, able to maintain a vacuum for some time, and having cut out a ring, a (Jig. 

 85), of tin foil, an inch and a half in internal diameter, and half an inch wide, paste it 

 upoQ the receiver, as at a {fig. 86) ; moreover, accommodate the receiver with its cam- 

 phor, as usual, and having exhausted, expose it to the direct rays of light, so that the 

 ring a shall be on the perihelion side. In the course of a short time that surface will 

 be found studded in various directions with crystals, as is to be expected ; but it will be 

 found that none of these crystals come within a certain distance of the ring, and that 

 not one is to be seen within the circle circumscribed by it. The ring, therefore, ex- 

 erts a kind of protecting action on the glass, hindering the deposition of crystals within 

 certain limits ; such a result is depicted \nfig. 87. 



478. This action of a ring, formed of good conducting materials, might be sup- 

 posed to arise either from its adding something to the surface of the glass, or taking 

 something away from the glass with which it is in contact. Or it might be imputed to 

 some change impressed on the ray of light. Take, therefore, a ring, a (Jig. 88), and 

 place it before the receiver, b, at a distance of half an inch, the ring being of the same 

 dimensions as in the last experiment ; it will be discovered that, although the ring does 

 not touch the glass, it still protects it, no crystals coming within a certain distance of 

 the regions overshadowed by the metal ; and even at a distance from the line of shadow 

 not a crystal is to be seen, nor any visible in the illuminated centre. 



479. Even after crystals have been formed on the surface of the jar, if it be placed 

 in the sunshine with a ring before it, as in the foregoing experiments, the ring will be 

 found not only to exert a protection on the glass, hindering any farther deposite, but 

 will even remove the crystals that are there. 



480. This is, indeed, a remarkable circumstance ; a part of the perihelion surface 

 is shaded from the sun, and thereby rendered cooler, yet the crystals deposite themselves 

 on the hottest surface, and avoid that where it is cold. We see, now, how it happens 

 that, in the experiment of admitting a column of light through a hole in a screen, no 

 crystalline deposite was effected ; the protecting agency of the metal, whatever its power 

 might be due to, hindered it. 



481. To give the particulars of one of these experiments. On the llth of July I 

 prepared an arrangement such as the foregoing : the thermometer in the shade was at 

 76 Fah., and in the sun at 99 Fah., distance of the ring from the jar half an inch, its 

 internal diameter three quarters, and width half an inch. After proper exposure the jar 

 was examined ; there were no crystals on that part opposite the central opening of the 

 ring, and the nearest crystal to the internal border was six tenths of an inch distant 

 from where the shadow was projected on the glass. 



482. Vapour of water exhibits similar phenomena ; a thin piece of tin foil in the form 

 of a cross, a ring, or any other shape, effectually prevents the deposite of water near it. 



483. Instead of placing the ring outside of the glass, now let it be placed on the 

 inside, as at a (^fig. 89), so that it may be within one eighth of an inch of the surface. 

 When the crystals have fully formed, it will be discovered that the ring has exerted 

 the same kind of protecting agency that it did when on the outside of the glass. 



484. Hitherto, a class of bodies has been tried as protectors which are without ex- 



