NATURE 
237 
THURSDAY, JANUARY 30, 1873 
THE INTERNATIONAL METRIC COMMIS- 
STON * 
7 | os continuation of the previous remarks upon the pro- 
ceedings of the Commission, we may now notice some 
of the more important scientific details of its operations. 
The material for constructing the new Standards, for 
which an alloy of pure platinum with 10 per cent. of iridium 
has been selected, is obviously a matter of primary import- 
ance. Before determining upon this metallic alloy, a series 
of experiments was made by the French section of the Com- 
mission. A material was needed, both for the metre and 
the kilogramme, that should as far as possible be un- 
alterable in its composition and molecular structure, in its 
form and dimensions, from the ordinary action of air, 
water, fire, or other chemical agents, or from mechanical 
_ forces to which it might be subject ; that would in fact 
possess physical properties rendering it invariable with 
time. It should be hard, elastic, and yet not difficult to 
work. It should at the same time be perfectly homo- 
geneous, so that all the Standards should be as nearly as 
possible identical in their material. And in order to 
lessen the unavoidable influences of variations of tempe- 
rature, it was obvious that a material was most desirable 
that would experience the jleast alteration in its dimen- 
sions from’ changes of temperature. Proceeding by an 
exhaustive process, the Commission decided against em- 
ploying any of the materials which have been hitherto 
adopted. Brass and alloys of copper did -not satisfy the 
requirements, and were rejected as liable to be injuriously 
affected by air and heat, and from being composed of 
different metals varying in their density and dilatation. 
_ Quartz, though satisfactory in many respects, was too 
fragile and bulky ; besides which there existed no pros- 
pect of obtaining it of sufficiently large dimensions and of 
the requisite purity. In addition to the objections attaching 
to quartz, glass was inadmissible by reason of the dis- 
turbing influences of moist air on its surface, and from its 
molecular condition as a tempered and crystallised body 
rendering it liable to changes from variation of tempera- 
ture which might affect the constancy of its density, 
expansion, and even length; for it was thought that a 
glass metre, like a steel metre, would ‘thus ‘become 
shorter in course of time. Even platinum, which was 
the best pure metal for the purpose, has the disadvantage 
of being too soft and too weak for a measuring bar. 
_ Combined, however, with a proper proportion of iridium, 
platinum satisfied all the conditions required either for a 
Standard metre or kilogramme. These two metals have 
the same system of regular crystallisation, the same 
_ density and rate of expansion, and when alloyed in proper 
_ proportions, they produce a perfectly homogeneous mate- 
rial. They are the two metals which of all others dilate 
the least by heat, and the proposed alloy of 10 per cent. 
of iridium has been proved to have as nearly as possible 
the same density and the same rate of expansion as the 
existing Metric Standards, thej J/étre and the K7/o- 
_ gramme des Archives. This fact alone is important as 
greatly facilitating the identity of the length and weight 
of the new Standards with those of the original proto- 
* Continued from p. 197. z 
No, 170—VoL. vit, 
types of the Metric System. Platinum-iridium has also 
been proved to be extremely hard and rigid, and to 
possess the greatest elasticity, as well as cohesion of 
resistance to fracture. At the same time, it is easily cut 
with a diamond, and it has been shown that lines ;,5, of 
a millimetre apart (or 0.00035 inch), so cut upon it, with 
the aid of a microscope, are perfectly regular, even when 
magnified from 300 to 600 times; 
The experiments of M. Regnault have shown that plati- 
num-iridium resists the penetration of absorbent gases, 
and further experiments made by the Commission prove 
that the influence of such gases can in no way cause any 
change, either in its volume or its weight. A more severe 
test had already been applied to platinum by M. Stas at 
Brussels. He subjected a platinum kilogramme suc- 
cessively to the action of alcohol, cold water, boiling water, 
drying in a vacuum, and heating in a red heat of from 
250° to 300° C., whilst guarded from flame ; and he ascer- 
tained by comparisons in moist air, at a temperature of 
15° C., with a platinum kilogramme not subjected to any 
of these conditions, that no change whatever had occurred 
in the weight of the kilogramme so treated. It was only 
requisite to allow a certain number of days, at most a 
fortnight, to elapse for the platinum to recover itself. 
Another important question was that of the form to be 
given to the new metre. The present Metre des Archives is 
a bar of platinum with a rectangular section, 25 millimetres 
wide, and 4 millimetres deep (or about 1 inch by } inch), 
It had been determined that the new metre, which was to 
be a standard @ ¢raits, or line-standard, should have its 
defining lines marked at mid-depth of the bar, on the 
same principle as our English standard of length, in order 
that the actual length of the measure should be as little 
as possible affected by any difference of temperature, and 
consequently of dilatation, between the upper and lower 
surfaces of the bar. But the Commission objected to the 
English mode of sinking cylindrical holes to the mid- 
depth of the bar, and tracing the defining lines on the 
plane surface of the bottom of these holes, as being not 
only inconvenient on many accounts, but also as inter- 
fering with the uniformity of structure of the bar during 
its whole length. The form of the new mdé¢re a traits 
mentioned in the resolutions as having been proposed by 
M. Tresca, one of the secretaries of the commission, who 
had given much study to the question, and laid an elabo- 
rate note upon the subject before the Commission, is of a 
very ingenious and entirely novel character. Its trans- 
verse section may be described as taken from the form of 
the letter X, if divided down the middle into two halves, 
and then joined by a band equal in thickness to the 
other parts (3 millimetres). By lowering the upper surface 
of this band to the mid-depth of the bar, it gives a con- 
tinuous plane, upon which not only the two defining lines 
of the metre can be cut, but also any intermediate lines 
that may be required as subdivisions of the metre. By a 
further slight reduction in the thickness of the lower half 
of the sectional figure, the defining lines will lie not only 
in the length of the neutral axis of the bar, but also in that 
of its centre of gravity. The dimensions of the bar itself 
when first constructed are to be 102 centimetres in length, 
and 2 centimetres square in section, and the bar is after- 
wards to be planed to the form decided upon. Its weight 
will thus be reduced to about 3} kilogrammes, and the 
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