306 Messrs. Haldane and Pembrey on an Improved Method 



though quite unmagnetic in large lumps, out of the crushed 

 rocks with magnets. Magnetic ore-separators are also said 

 to be used on a practical scale. 



In conclusion I may mention that M. Heusler, of the 

 Isabellenhtitte, Dillenburg, kindly sent me an alloy which he 

 defined as follows : — 93-93*5 per cent. Mn, 1-1*2 per cent. 

 Fe, *2-*5 per cent. Si, and carbon. This gave not the 

 slightest trace of an attraction by the pole of a steel magnet 

 or of the Ruhnikorff, whether in a piece or pounded almost 

 to dust. 



I beg to tender my best thanks to Prof. F. Kohlrausch, in 

 whose laboratory these experiments were carried out. 



Phys. Inst, of Strasburg Univ., 

 Nov. 1, 1889. 



XXXIII. An Improved Method of Determining Moisture and 

 Carbonic Acid in Air. By J. S. Haldane, M.A., M.D., 

 and M. S. Pembrey, B.A., Fell Exhibitioner of Christ 

 Church, Oxford. {From the Physiological laboratory, 

 Oxford.) * 



WE were originally led to undertake the present investi- 

 gation by difficulties experienced in measuring the 

 respiratory exchange of oxygen and carbonic acid in animals. 

 In the method which we wished to employ the results depend 

 on an accurate determination of the difference in the per- 

 centage of moisture and carbonic acid in the air entering and 

 that leaving a ventilated chamber in which the animal is 

 placed. As the ventilation current is large, the difference to 

 be measured is a small one, and errors of analysis are corre- 

 spondingly important, especially in short experiments. 



Although the object of our work was thus originally physio- 

 logical, w r e have been guided chiefly by regard to the wider 

 applications, particularly in meteorology, of the methods in 

 question. 



I. The Determination of Moisture. 



Of the various methods in use for determining moisture in 

 air the " chemical " method is generally acknowledged to be 

 the most accurate when properly carried out. This method 

 consists in the aspiration of a known volume of the air 

 through one or more weighed tubes filled with a substance 

 which absorbs moisture, such as anhydrous phosphoric acid 

 or pumice soaked in sulphuric acid. The increase in weight 

 of the absorption- tubes gives the weight of moisture contained 

 •in the air. The great disadvantage of this method as used 



* Communicated bj Prof. Odling, F.K.S. 



