ATOMIC WEIGHTS ASTON. 



183 



and the operation of section will consist of three cuts at right angles 

 to each other, dividing the original cube into eight similar bodies 

 each of half the linear dimensions and one-eighth the weight. Thus 

 the first cube will have 5 cm. sides and weigh 1.42 kilograms the 

 second will weigh 178 grs., the fourth 2.78 grs., and so o n r Diminu- 

 tion in the series is very rapid and the result of the ninth operation 

 is a quantity of lead just weighable on the ordinary chemical balance. 

 The results of further operations are compared with suitable objects 

 and a scale of length in Figures 1, 2, and 3. The last operation pos- 

 sible, without breaking up the lead atom, is the twenty-eighth. The 

 twenty-sixth cube is illustrated in Figure 3. It contains 64 atoms, 

 whose size, distance apart, and general arrangement can be repre- 

 sented with considerable accuracy, thanks to the exact knowledge de- 

 rived from research on X rays and specific heats. On the same scale 

 are represented the largest atom, caesium, and the smallest atom, car- 

 bon, together with molecules of oxygen and nitrogen, at their aver- 

 age distance apart in the air, and the helical arrangement of silicon 

 and oxygen atoms in quartz crystals discovered by X-ray analysis. 

 The following table shows at what stages certain analytical methods 

 break down. The great superiority of the microscope is a noteworthy 

 point. 



Cube. 



9 

 14 

 15 

 18 

 24 

 28 

 Atom. 



Side in cms. 



.0195 

 6.1 xlO- 4 

 3. 05 x 10- 

 3.8 xlO' 8 

 6.0 xlO" 7 

 3. 7. x 10- 8 

 3.0 xlO" 8 



Mass in grms. 

 8.5 xlO" 5 



2. 58 x 10-" 



3. 22 x lO" 10 

 6. 25 x 10- 13 



2. 38 x 10- 18 

 5. 15 x 10' 22 



3. 44 x 10" 22 



Limiting analytical method. 

 Ordinary Chemical Balance. 

 Quartz Micro-balance. 

 Spectrum Analysis (Na lines). 

 Ordinary Microscope. 

 Ultra Microscope. 



Radioactivity. 



Just as any vivid notion of the size of the cubes passes out of our 

 power at about the twelfth— the limiting size of a dark object visible 

 to the unaided eye— so when one considers the figures expressing the 

 number of atoms in any ordinary mass of material, the mind is stag- 

 gered by their immensity. Thus, if we slice the original decimeter 

 cube into square plates one atom thick the area of these plates will 

 total one and one-quarter square miles. If we cut these plates into 

 strings of atoms spaced apart as they are in the solid, these decimeter 

 strings put end to end will reach 6.3 million million miles, the dis- 

 tance light will travel in a year, a quarter of the distance to the near- 

 est fixed star. If the atoms are spaced but one millimeter apart the 

 string will be three and a half million times longer yet, spanning the 

 whole universe. 



Again, if an ordinary evacuated electric light bulb were pierced 

 with an aperture such that one million molecules of the air entered 

 per second, the pressure in the bulb would not rise to that of the 



