46 DB. LATJDEIl LINDSAT OK CHEMICAL BEACTIOK 



lichenologists as a specific character ; and the reason why such 

 recognition has fallen into desuetude is probably to be sought in 



one of two causes — either (1) 



(2) 



cTiemical characters could not be trusted in the determination 

 and classification of species ! 



V. Tinctura lodi, otherwise Tincture of Iodine, containing a 

 proportion of iodide of potassium as a solvent, — Whilst chemists 

 tell us that the reaction of starch and iodine is so delicate as to 



be discernible in water containing ^^^^ part of its weight of 



iodine, they also point out that the reaction is apt to be inter- 

 fered with by a number of chemical obstacles. Moreover, if 

 eJiemists are right in asserting that only the ordinary form of 

 starch gives a blue reaction with iodine, lichenologists must be 

 wrong in their supposition that what they call " hymenial gela- 

 tine'' is, in all cases, lichenine^l The irregularities in there- 

 action of iodine in Lichens is sufficiently explained by the dif- 

 ferent reactions which iodine gives with different modifications of 

 starch, and by the aptitude of these reactions to be disturbed by 

 a number of trivial chemical causes. Ny lander recommends the 

 following formula for making the iodine- test solutionf : — Iodine 

 1 gr-, iodide of potassium 3 grs., distilled water ^ oz. The solu- 

 tion should be kept from light in a black bottle, or in one covered 



with paper- In testing microscopically, it is sufficient to apply 

 a drop to the edge of the thin glass covering the dissection, 



under which it will diffuse itself in the water containing the ob- 

 ject. I am not aware, however, of any advantage this solution 

 possesses over our officinal tincture of iodine, diluted with water 

 to such extent that the liq[uid has only a pale sherry colour. For 

 all practical pm^oses, I have found, for 20 years, the latter solu- 

 tion sufficient. I have used the iodine test chiefly in microsco- 

 pical analysis of the hymenium, sometimes also of the thallus, in 

 order to the detection of starch in some of its modifications ; and 

 I pointed out its usefulness in my text-book on the British 

 Lichens (1856, p. 111). I have also employed it as a supposed 

 differential test between Lichens and Fungi, in which respect, 



Vide p. 40. 

 t '* On the Reaction of Iodine in Lichens and Fungi/' " Not. Lichenologicse " of 

 Leighton, Ann. Nat. Hist Jan. 1866, p. 59, March 1866, p. 190, Aug. 1866, p. 106; 

 also footnote, Journ. of Linn. Soc. voL cUat p. 360, and quoted in * Science 

 Gossip,' 1866, p. 42. 



