362 



Mr. W. Crookes on the Measurement of the 



with those obtained here. Without presuming to say that he has satis- 

 factorily solved all difficulties, the writer believes that he has advanced 

 some distance in the right direction, and pointed out the road for further 

 improvement. 



Before deciding upon a standard light, experiments were made to ascer- 

 tain whether the electric current could be made available. Through a 

 coil of platinum wire, so as to render it brightly incandescent, a powerful 

 galvanic current was passed, and its strength was kept as constant as 

 possible by a thick wire galvanometer and rheostat. To prevent the 

 cooling action of air-currents, the incandescent coil was surrounded with 

 glass ; and it was hoped that by employing the same kind of battery, and 

 by varving the resistance so as to keep the galvanometer-needle at the 

 same deflection, uniform results could be obtained. In practice, however, 

 it was found that many things interfered with the uniformity of the re- 

 sults, and the light being much feebler than it was advisable to work with, 

 this plan was deemed not sufficiently promising, and it was abandoned. 



The method ultimately decided upon is the following : — Alcohol of sp. 

 gr. 0*805, and pure benzol boiling at 81° C, are mixed together in the 

 proportion of 5 volumes of alcohol and 1 of benzol. This burning fluid 

 can be accurately imitated from description at any future time and in any 

 country ; and if a lamp could be devised equally simple and invariable, 

 the light which it would yield would, it is presumed, be invariable. This 

 difficulty the writer has attempted to overcome in the following manner. 



A glass lamp is taken of about two ounces capacity, the aperture in the 

 neck being 0*25 inch diameter ; another aperture at the side allows the 

 liquid fuel to be introduced, and, by a well-known laboratory device, tbe 

 level of the fluid in the lamp can be kept uniform. The wick-holder con- 

 sists of a platinum tube 1*81 inch long and 0*125 inch internal diameter. 

 The bottom of this is closed with a flat plug of platinum, apertures being 

 left in the sides to allow free access of spirit. A small platinum cup 

 0*5 inch diameter and 0*1 inch deep is soldered round the outside of the 

 tube 0*5 inch from the top, answering the threefold purpose of keeping 

 the wick-holder at a proper height in the lamp, preventing evaporation of 

 the liquid, and keeping out dust. The wick consists of fifty-two pieces 

 of hard-drawn platinum wire, each 0*01 inch in diameter and 2 inches 

 long, perfectly straight, and tightly pushed down into the platinum holder, 

 until only 0*1 inch projects above the tube. The height of the burning 

 fluid in the lamp must be sufficient to cover the bottom of the wick- 

 holder : it answers best to keep it always at the uniform distance of 1*75 

 inch from the top of the platinum wick ; a slight variation of level, 

 however, has not been found to influence the light to an extent appre- 

 ciable by our present means of photometry. The lamp with reservoir of 

 spirit thus arranged, with the platinum wires parallel, and their projecting 

 ends level, a light is applied, and the flame instantly appears, forming a 

 perfectly shaped cone 1*25 inch in height, the point of maximum bril- 



