490 Transactions. — CJiemistry and Physics. 



Deviation produced with microscope-slip 1-6 mm. in 



thickness {t = 5*9 mm.) ... = + 47' 



„ with microscope-shp 1 mm. in 



thickness {t = 6-5 mm.) ... = + 49' 



„ with twomicroscope-shps, 2mm. 



and 16 nmi. in thickness 

 {t = 3-9 mm.) ... ... = + 36' 



„ by thoroughly mixed liquids in 



the tank) " ... ... = - 3' 



Thickness of liquid layer in tank = 7"5 mm. 



On the emission theory such deviation is unaccountable, 

 if our mcidence be normal. Hence we may regard this as an 

 experiinentum crncis for the wave theory. I have been unable 

 to find any previous account of it. The experiments were 

 made at Canterbury College in August, 1900. 



Akt. XLVI. — The Physical Properties of Kauri-gum. 



By J. S. S. CooPEE, M.A., B.Sc. 



[Read before tlie Otago Instihite, 13th October, 1903.] 



[Preliminary Communication.] 



The fossil gum from the kauri has many well-known proper- 

 ties peculiar to itself. It was thought worth while to make it 

 the subject of a systematic physical investigation. Although 

 this work is far from finished, it seems best to publish what 

 iew results have been obtained, and to supplement them at 

 a later date. The samples used were of good quality, clear- 

 yellow or milky in appearance, and in some cases of con- 

 siderable size — up to a pound or so in weight. They were 

 obtained from Messrs. Heather, Eoberton, and Co., of Auck- 

 land. 



1. General Propekties. 



The specific gravity varies with the milkiness — i.e., with 

 the vesicularity — so that the specific gravity of the pure 

 substance would appear to be very constant. 



Specific giavity of clear sample ... 1-076 



„ „ milky sample ... 1-049 



„ „ very milky sample 1037 



On being heated in the open the gum rapidly decomposes, 

 and may be destructively distilled into a very large number of 

 fractions. On heating under pressure in a confined space the 

 gum may be melted as a whole, and then resolidifies nearly 

 unchanged, but slightly more dense. During the heating to 



