( 1097 ) 
taken into account. Hence, in the very nature of the determinations 
theinselves there is cause for many uncertainties. In the meantime 
however, it seems best to make a beginning by assuming that the 
ordinary gas laws may still be applied at these densities, and to 
postpone the application of corrections for the deviations that should 
follow according to the law of corresponding states and for possible 
condensation of vapour on the walls, ete., until experiments have 
been completed which will afford an estimate of these corrections. 
In this way one can at least attempt to obtain data concerning cer- 
tain thermal properties of helium. I mentioned in Comm. N°. 112 
that at that time I had not been successful in overcoming the diffi- 
culties that are always encountered when making measurements with 
a thermometer, built on the principle of the one that has hitherto 
been used, in the immediate neighbourhood of apparatus that are 
used for the preparation of liquid helium. These difficulties have 
not yet been wholly removed. The necessity for simplicity and ease 
of manipulation of the apparatus, and the fact that the thermometric 
measurements should be independent of all vibration and all disturbances 
are difficult to reconcile. But still, it would appear that the tempe- 
ratures obtained may be relied upon to within 1/10th of a degree. 
The part of the helium thermometer that serves for the adjust- 
ment of the constant volume, and for the reading of the pressure 
is shown on the right hand side of Plate I. Its arrangement is simi- 
lar to that of the hydrogen thermometer shown on Plate | of Comm. 
N°. 95e (Oct. 1906) when this is being used for measuring hydro- 
gen temperatures ; part of the lettering is chosen so as to correspond 
with that of the latter plate. On account of the smallness of the 
pressures to be measured at the helium temperatures the space above 
the mercury in the adjustable double manometer tube / and f, is 
evacuated, and, to make quite certain, they are connected to an 
evacuated tube / filled with charcoal and immersed in liquid air. 
The manometer tube on which the pressures are read off, has, as 
well as the adjusting tube, a small steel point, so that the difference 
of jievel between the two mercury menisci may be obtained with 
greater accuracy. The base eg which carries the point fa is slitted, 
as can be seen in the figure. before making any adjustments the 
tap ys, is closed, and it is opened to allow communication between 
the mercury in the reading tube and that in the adjusting tube of 
the manometer only after the mercury meniscus in the adjusting 
tube has heen brought to the level of the point f, in the reading 
tube by opening Ay, and moving the double manometer tube up and 
down until this is accomplished. Then to proceed to an adjustment 
