132 THE CELL 



box. Ultra-violet rays have a marked effect on living organisms. 

 They kill, e.g. the tubercle bacillus is killed by ultra-violet light 

 and lupus is cured by projection of the Finsen arc on the growth. 

 Change of temperature may exert either a positive or a negative 

 effect, the animalcule avoiding the abnormal. That is, too high 

 or too low a temperature exerts negative thermotaxis. Non- 

 living matter shows irritability. We have seen how sensitive 

 colloids are to slight alterations in their environment. They 

 exhibit chemiotaxis and galvanotaxis very markedly. Even 

 inorganic matter may respond to stimulation. Lillie has demon- 

 strated this in the case of iron. A piano wire which has been 

 dipped in concentrated nitric acid and then suspended in dilute 

 nitric acid will show changes if " stimulated " mechanically, 

 chemically, or electrically. The irritability of living matter is, 

 according to Verworn, of a specific type and is thus indicative 

 of life. 



(c) Ingestion and excretion are phenomena exhibited by all 

 living cells. Nutrient material is taken from the environment, 

 prepared, used, and the non-utilisable rest is forced out. Amoeba 

 engulfs food and forms a vacuole in which will be found food and 

 water. Into this vacuole are secreted digestive enzymes which 

 reduce the ingested material if possible from the colloidal to the 

 crystalloidal state. It then passes into the protoplasm and 

 the undigested residue is forcibly excreted by contraction of the 

 vacuole. These processes all have their physico-chemical counter- 

 parts. A drop of chloroform will reject a piece of capillary glass 

 tubing forced into it. If the glass be coated with shellac it will 

 be drawn into the chloroform, the shellac dissolved from it and 

 then the clean glass is expelled from the interior of the chloroform 

 to the surface. 



(d) Growth is not a property characteristic of living matter. 

 Leduc has taught us that by osmosis life-like forms may be 

 produced which grow. 



(e) Electric Phenomena. The electrical power generated by 

 living matter has always been a subject of interest and of amaze- 

 ment. Quite apart from such animals as possess electrical organs, 

 e.g. the electric eel which can generate an E.M.F. of several 

 hundred volts, every living animal, in fact every living cell, pro- 

 duces electromotive forces. The ordinary potential differences 

 observed in living matter may seldom reach 0-1 volt, but everyone 

 knows that if n small units are connected in series, the resultant 

 voltage is n times the voltage of the single unit. Dissection of an 



