10 On some Improvements in the 



ammonia and a small quantity of the double tartrate produces no 

 change, but on adding a small piece of the ferrous salt, about ^Vth 

 of an inch in diameter, and carefully stirring, so as to mix without 

 much exposure to the air, these bands gradually fade, and are replaced 

 by the single broad and fainter band of deoxidized haemoglobin. 

 When stirred up so as to expose well to the air, the two original 

 bands of oxidized haemoglobin can be seen again. On gradually 

 adding a little citric acid, until the colour begins to change, these 

 bands slowly fade away ; and, if the amount of blood was consi- 

 derable, a faint band would make its appearance in the red. When 

 previously deoxidized, this solution may be turbid, but not so as to 

 interfere with the result. The addition of excess of ammonia makes 

 all clear again, but does not restore the original bands, or only to a 

 slight degree, thus showing that a permanent change is produced 

 by citric acid — the hasmoglobin is changed into haematin. This 

 alone serves to distinguish blood from by far the greater number of 

 coloured substances, which after being changed by acid, are restored 

 by alkalis to the original state. On deoxidizing with the ferrous 

 salt, we obtain the well-marked spectrum of deoxidized hsematin, 

 with one very dark and another much fainter band in the green, 

 almost or quit-e invisible when the quantity is small. If too much 

 citric acid or double tartrate had been added, this solution might be 

 turbid; but, if all had been properly managed, it would be quite 

 clear. Since the deoxidization takes place rather slowly, especially 

 in cold weather, it is well to slightly stir up the ferrous salt at the 

 bottom, completely fill up the cell, cover it with a piece of thin 

 glass, remove the excess of liquid with blotting paper, and mix the 

 solution by turning the tube upside down, over and over again. On 

 reoxidizing the solution by stirring, the bands of deoxidized haematin 

 disappear, and the two bands of haemoglobin will probably be recog- 

 nized, owing to citric acid not changing the original merely into 

 hffimatin, but also giving rise to some methaemoglobin. The whole 

 of these facts may be seen with a single cell, containing about y^oth 

 of a grain of blood, and any experimenter should become quite 

 familiar with them before applying this method to suspected stains 

 in cases of importance. Very faint bande are best seen by lamplight. 

 On exposure to the air in a damp place, a blood-stain may be 

 completely decomposed by the growth of mould, but when not thus 

 destroyed it is partly altered into hfematin. If, however, kept dry, 

 the haemoglobin gradually changes into a variable mixture of methae- 

 moglobin, haematin, and a brown substance not yet much studied. 

 This change takes place far more rapidly in the acid atmosphere of 

 towns and houses, especially when gas is burned, than in the open 

 country ; but it does occur even in the purest air, and in glass tubes 

 hermetically sealed. The presence of a weak aoid in perspiration 

 may also cause a stain on a worn garment to be completely changed 



