August 7, 1879] 



NATURE 



343 



the material into iridiate and rhuthenate of potash, and the 

 oxidation of the iron ; when cold, the mixture is treated 

 with cold distilled water. The iridiate of potash of a 

 blue tinge will remain as a deposit almost insoluble in 

 water, more especially if slightly alkaline, and also the 

 oxide of iron. 



This precipitate must be well washed with water 

 charged with a little potash and hypochlorite of soda 

 until the washings are no longer coloured, and then seve- 

 ral times with distilled water. 



The blue powder is then mixed with water strongly 

 charged with hypochlorite of soda, and allowed to remain 

 for a time cold, then warmed in a distilling vessel, and 

 finally brought up to boiling point until the distillate no 

 longer colours red, weak alcohol acidulated with hydro- 

 chloric acid. 



The residue is again heated with nitre and potash water 

 charged with hypochlorite of soda and chlorine, until 

 the last trace of ruthenium has disappeared. 



Further, to carry out the purification, the blue powder 

 {oxide of iridium) is re-dissolved in aqua regia, evapo- 

 rated to dryness, re-dissolved in water, and filtered. 



The dark-coloured solution thus obtained is slowly 

 poured into a concentrated solution of soda and mixed 

 with hypochlorite of soda, and should remain as a clear 

 solution without any perceptible precipitate, and subjected 

 in a distilling apparatus to a stream of chlorine gas, 

 should not show a trace of ruthenium when hydrochloric 

 acid and alcohol are introduced into the receiver. In this 

 operation the chlorine precipitates the greater part of the 

 iridium in a state of blue oxide, which, after being col- 

 lected, washed, and dried, is placed in a porcelain or 

 glass tube, and subjected to the combined action of oxide 

 of carbon and carbonic acid obtained by means of a 

 mixture of oxalic with sulphuric acid gently heated. 



The oxide of iridium is reduced by the action of the 

 gas leaving the oxide of iron intact, the mass is then 

 heated to redness with bi-sulphate of potash (which will 

 take up the iron and any remaining trace of rhodium), 

 and after subjecting it to many washings with distilled 

 water, the residue is washed with chlorine water to 

 remove any trace of gold, and finally with hydrofluoric 

 acid, in order to take out any silica which might have 

 been accidentally introduced with the alkalies employed 

 or have come off the vessels used. 



The iridium after calcination at a strong heat in a char- 

 coal crucible, is melted into an ingot. 



Alloy of Iridio-Platimim 



Operating upon a charge of 450 ounces of platinum and 

 55 ounces of iridium, I commenced by melting these 

 metals together and casting into an ingot of suitable 

 shape, which I then cut into small pieces with hydraulic 

 machinery. After re-melting and retaining in a molten 

 condition under a powerful flame of oxygen and common 

 gas for a considerable time, I re-cast and forged the mass 

 at an intense white heat under a'steam hammer, the highly- 

 polished surfaces of which were cleaned and polished after 

 each series of blows — when sufficiently reduced the alloy 

 was passed through bright polished steel rollers, cut into 

 narrow strips, and again slowly melted in a properly- 

 shaped mould, in which it was allowed to cool. I thus 

 obtained a mass of suitable shape for forging, perfectly 

 solid, homogeneous, free from fissures or air-holes, and 

 with a bright and clean surface. 



A piece cut from the end of a mass so prepared, was 

 presented to the French Academy of Science, and gave 

 the following results : — 



Weight in air liS'SgSgrms. 



,, water Iir469 ,, 



Showing a density of 2i'5i6 „ 



thus proving that the necessary processes of annealing at 

 a high temperature had caused it to resume its original 

 density. 



The analysis gave- 



99-90 4-644 



Density at zero, calculated after No. I analysis 21'SIO 



Density at zero, calculated after No. 2 ,, 21-515 



which coincide perfectly with the practical results 

 obtained." 



MM. Deville and Mascart find the coefficient of 

 dilatation to be from 0° to 16^ C. 0-00002541. 



As we have already pointed out, work on which the 

 accuracy of standards depends is of the highest import- 

 ance, and Mr. Matthey is therefore to be congratulated 

 on the success of his labours. 



THE INFLUENCE OF THE TRANSVERSE 

 DIMENSIONS OF ORGAN PIPES ON THE 

 PITCH 



IN Nature, vol. xix. p. 172, Mr. Ellis gives, on the autho- 

 rity of M. Cavailld-Coll, a rule determining a point of 

 some interest in regard to organ-pipes. All those who are 

 accustomed to organs know that the theoretical rule which 

 makes the vibration-number of the note sounded vary 

 inversely as the length of the pipe, does not hold correctly 

 in practice, as the pitch is influenced by the transverse 

 dimensions. A pipe of "large scale," i.e., of large 

 diameter, will speak a lower note than one of "small 

 scale," the length of the tube being in both cases the 

 same. I am not aware that this fact has been explained 

 in acoustical works, or any rule given for the variation. 



Mr. Ellis's formula provides for this, so far as cylin- 

 drical pipes are concerned, and he has found it to agree 

 well with experiment. There is a misprint in his equation, 

 which at first sight renders it somewhat obscure, and in 

 correcting this I will venture to present M. Cavailld-CoU's 

 investigation more completely, as it was expressed by him 

 in a paper presented to the Academy of Sciences many 

 years ago, and a copy of which he was good enough to 

 give me. 



After calling attention to the theoretical rule, he remarks 

 that the departure from it is due to the influence of the 

 mouth of the pipe, i.e., the rectangular opening at the 

 lower end of the tube. He made many experiments to 

 determine the effect of this, and came at length to the 

 result that in open pipes of rectangular section the effective 

 length of the pipe was equal to the length of the sound- 

 wave due to its note [or the half wave-length according 

 to our mode of calculation] diminished by tzuice the internal 

 depth of the tube. By the " depth " is meant the transverse 

 dimension from front to back ; the other transverse dimen- 

 sion, the -width, appearing to be of no consequence. 



Thus, if ^" represent the velocity of sound, K the num- 

 ber of vibrations per second {single ones, according to the 

 French mode of calculation), L the length of the pipe, 

 taken from the lower edge of the mouth to the end of the 

 tube ; and P the internal transverse depth, then— 



