June 9, 1898] 



NA TURE 



139 



The figures show how largely the physical and mechanical 

 properties depend on the relative proportion of gutta and resin, 



i.e. the ratio — . The temperatures given as those at which the 



R 

 material softens and at which it becomes pliable, have only a 

 relative value, as they apply to the particular method of testing 

 here employed, but for comparative purposes they are most 

 valuable. 



rigid to resist the pressure of the stud in the apparatus used for 

 determining the softening temperature, th^ water surroundihg 

 the strip being maintained at 75° K 



With a view to investigating the action of oxygen on cleaned 

 gutta percha more thoroughly than had hitherto been done, Dr. 

 Obach conducted an exhaustive series of experiments having for 

 their special object a direct comparison of the avidity with which 

 the different " brands" of gutta percha absorbed oxygen under 



Table II. 



-Insulation and Induction per Cube Knot with Low and High Percentage of Water. 

 (Abstracted from larger table. ) 



The softening temperature is determined as follows : — A thin 

 sheet of the gutta percha to be tested is very slowly heated in 

 a water bath, and a small stud from time to time brought to bear 

 upon it with a definite pressure. As soon as the stud leaves a 

 permanent impression on the surface of the sheet, the tem- 

 perature of the water is noted and recorded as the "softening 

 temperature." 



The temperature at which the material becomes pliable is thus 



similar conditions. For this purpose small spheres, of 2 cub. 

 cm. contents and 8 sq. cm. superficial area, were enclosed in 

 glass tubes filled with oxygen and inverted over mercury troughs. 

 The tubes had a capacity of about 30 cub. cm., and each contained 

 two spheres. They were refilled as soon as the composition of the 

 residual gas approached that of the air, the oxygen used con- 

 taining about 7 per cent, of nitrogen. The mercury troughs 

 were placed outside a window on the south front of the labora- 



Table III. — Chemical Composition, Physical, Mechanical and Electrical Properties of the First Grades of Twelve Different 



^^ Brands" of Gutta Percha. 

 (Abstracted from larger table. ) 



A^<,/<.— Each series of figures in this table represents the .average result obtained with a number of individual lots of the particular brand. 



determined : — A strip of the material of definite dimensions is 

 held vertically in a bath of water ; the upper end of the strip is 

 attached to a cord, passing over a pulley and carrying a known 

 weight, the strip being thus subjected to a constant tension. 

 The temperature of the water at the moment when the weight is 

 .able to pull the strip asunder, is taken as that of " pliability." 



The "time of hardening" is that taken by the material, 

 heated to the temperature of pliability, to become sufficiently 



NO. 1493, VOL. 58] 



tory exposed to full sunshine. The experiment extended over 

 twenty-four weeks, and during that period the total amount of 

 bright sunshine amounted to 680 hours. 



The smoothed curves, given in Fig. 4, show the average amount 

 of oxygen in cub. cm. absorbed by each of the four different 

 groups of materials and for comparison, also that absorbed by 

 gutta percha obtained from leaves and by balata. As in the case 

 of the experiments on the absorption of water, two sets of curves 



