THE COLOURED BLOOD-CORPUSCLES. 29 



Take care not to brush the cell with anything rougher than 

 a camel's- hair pencil, to avoid injuring the lines. 



Each square has an area of yj^ mm., so that 10 squares have 

 an area of y 1 ^ mm. As the cell is 4 mm. deep, the volume of 

 blood in 10 squares is yV x i = "sV c - mi]a ' On counting the 

 number of corpuscles in 10 squares, and multiplying by 50, this 

 will give the number in 1 c.mm. of the diluted blood. On 

 multiplying this by -^^j we get the number in 1 c.mm. before 

 dilution. Thus we arrive at the reason why we multiply the 

 number in 10 squares by 10,000 to get the number of cor- 

 puscles in 1 c.mm. of blood. 



HAEMOGLOBIN AND ITS DERIVATIVES. 



3. Preparation of Haemoglobin Crystals. 



(a.) Rat's Blood. Place a drop of the defibriiiated rat's 

 blood provided for you on a slide, add three or four drops 

 of water, mix, and cover with a cover-glass. Examine 

 the slide with a high power of the microscope ; after a 

 few minutes, especially at the edges of the preparation, 

 small crystals will begin to form, and gradually grow larger. 

 The crystals are those of oxy-hsemoglobin, and have the form 

 of thin rhombic plates, disposed singly or in groups. 



(b.) Dog's Blood. To 15 cc. of the defibrinated dog's blood 

 provided for you, add, drop by drop, 1 cc. or so of ether, 

 shaking the tube after each addition of ether. By this 

 means the blood is rendered laky, a condition which is 

 recognised by inclining the tube, and observing that the 

 film of blood left on it, on bringing the tube to the vertical 

 again, is transparent. Add no more ether, but place the 

 tube in a freezing-mixture of ice and salt ; as the tempera- 

 ture falls, crystals of haemoglobin separate. If the crystals 

 do not separate at once, keep the tube in the freezing- 

 mixture for one or two days. Examine some of the crystals 

 under the microscope. 



4. Ozone Test for Haemoglobin. Mix some freshly-prepared 

 alcoholic solution of guaiacurn with ozonic ether ; the mixture 

 becomes turbid, and on adding even a dilute solution of hsemo- 



