joo 



NATURE 



[Fed, I, 1877 



2. The successive swings and final deflection when a 

 candle shines on one end of the blackened bar. 



3. The appearance of the induction spark between the 

 platinum wires. 



I measures the viscosity of the gas ; 2 enables me to cal- 

 culate the force of radiation of the candle ; and 3 enables 

 me to form an idea of the progress of the vacuum according 

 as the interior of the tube becomes uniformly luminous, 

 striated, luminous at the poles only, or black and non- 

 conducting. The movement is started by rotating the 

 whole apparatus through a small angle, and the observa- 

 tion consists in noting the successive amplitudes of 

 vibration when the instrument is left to itself, a mirror 

 and spot of light being eniployed for this purpose. The 

 amplitudes form a decreasing series, with a regular 

 logarithmic decrement. Up to the point at which the 

 vacuum is apparently equal to a Torricellian vacuum, the 

 logarithmic decrement is nearly constant ; but as the 

 exhaustion proceeds beyond this point, it becomes smaller, 

 and the force of repulsion approaches a maximum ; when 

 the logarithmic decrement is about one fourth of what it 

 was at the commencement, the force of repulsion begins 

 to diminish, and at much higher exhaustions it nearly 

 ceases. 



I have experimented with different gases in the appa- 

 ratus, and by means of a McLeod gauge attached to a 

 mercury-pump, I have been able to measure the atmo- 

 spheric pressure at any desired state of exhaustion. From 

 the results of the measurements of the force of repulsion 

 and of the viscosity of the residual gas, I have plotted the 

 observations in curves, which show how the viscosity of 

 the residual gas is related to the force of repulsion 

 exerted by radiation. 



I have supposed my scale to be 1,000 metres long, and 

 to represent one atmosphere. Each millimetre, therefore, 

 stands for the millionth 0/ an atmosphere. 



When the residual gas is air, the viscosity, measured 

 by the logarithmic decrement of the arc of oscillation, is 

 practically constant up to an exhaustion of 250 millionths 

 of an atmosphere, or o'lg millim. of mercury, having only 

 diminished from o'i26 at the normal pressure of the 

 atmosphere, to o'ii2. It now begins to fall off and at 

 01 of a millionth of an atmosphere the logarithmic decre- 

 ment has fallen to about o 01. Simultaneously with this 

 decrease in the viscosity, the force of repulsion exerted 

 on a black surface by a standard light varies. It in- 

 creases very slowly till the exhaustion has risen to about 

 70 millionths of an atmosphere ; at about 40 millionths 

 the force is at its maximum ; and it then sinks very 

 rapidly till at o*i millionth of an atmosphere it is less 

 than one-tenth of its maximum. 



When the residual gas is oxygen the logarithmic decre- 

 ment is o"i26 at the atmospheric pressure, and at 2 mil- 

 lionths of an atmosphere it is 002. The force of repulsion 

 in oxygen increases very steadily up to an exhaustion of 

 about 40 millionths of an atmosphere ; it is at its maxi- 

 mum at about 30 millionths, and thence declines very 

 rapidly. 



It is not necessary to get so high an exhaustion with 

 hydrogen as with other gases to obtain considerable 

 repulsion. The viscosity at the normal pressure is measured 

 by a logarithmic decrement of o"o63 ; at 50 millionths it 

 is 0'046, when it rapidly sinks. The force of repulsion is 

 very great in a hydrogen vacuum being, in comparison 

 with the maximum in an air vacuum as 70 to 41. 



Carbonic acid has a viscosity of about 001 at the 

 normal pressure, being between air and hydrogen, but 

 nearer the former. The force of repulsion does not rise 

 very high and soon falls off. 



A long series of observations have been taken, at dif- 

 ferent degrees of exhaustion, on the conductivity of the 

 residual gas to the spark from an induction coil. Work- 

 ing with air I find that at a pressure of about 40 millionths 

 ot an atmosphere, when the repulsive force is near its 



maximum, a spark, whose strikin^g distance at the normal 

 pressure is half an inch, will illuminate a tube having 

 terminals 3 millimetres apart When I push the ex- 

 haustion further the ^-inch spark ceases to pass, but 2 

 I -inch spark will still illuminate the tube. As I get 

 nearer to a vacuum more power is required to drive the 

 spark through the tube, but, at the highest exhaustions, I 

 can still get traces of conductivity when an induction 

 coil, actuated with five Grove's cells, and capable of 

 giving a 6-inch spark, is used. 



When so powerful a «park is employed it sometimes 

 happens that the glass is perforated, thus causing a very 

 slight leakage of air into the apparatus. The logarithmic 

 decrement now slowly rises, the repulsive force of the 

 candle increases to its maximum and then slowly dimi- 

 nishes to zero, the logarithmic decrement continuing to 

 rise till it shows that the internal and external pressure 

 are identical. 



In preparing experimental radiometers I prefer to 

 exhaust direct to one or two millionths of an atmosphere. 

 By keeping the apparatus during the exhaustion in a hot 

 air-bath heated to about 300° C. for some hours, the 

 occluded gases are driven off from the interior surface of 

 the glass and the fly of the radiometer. The whole is 

 then allowed to cool, and attenuated air from the air-trap 

 is put in in small quantities at a time, until the McLeod 

 gauge shows that the best exhaustion for sensitiveness is 

 reached ; if necessary, this point is also ascertained by 

 testing with a candle. In this manner, employing hydro- 

 gen instead of air for the gaseous residue, and using 

 roasted mica vanes at an angle with the axis, I can get 

 very considerably increased sensitiveness in radiometers. 

 I am still unable, however, to get them to move in moon- 

 light, while my sensitive torsion balance does easily. 



I have tried many experiments with the view of putting 

 the theory I have referred to in my former paper 

 (Nature, vol. xv. p. 224) to a decisive test. The re- 

 pulsive force being due to a molecular disturbance 

 causing a reaction between the fly and the glass case 

 of a radiometer, it follows that, other things being 

 equal, the fly should revolve faster in a small bulb 

 than in a large one. I therefore constructed a double 

 radiometer which shows this fact in a very satisfactory 

 manner. It consists of two bulbs, one large and the other 

 small, blown together so as to have a wide passage be- 

 tween them. In the centre of each bulb is a cup, held in 

 its place by a glass rod, and in the bulbs is a small four- 

 armed fly with roasted mica discs blacked on one side. 

 The fly can be balanced on either cup. In the smaller 

 bulb there is about a quarter of an inch between the 

 vanes and the glass, whilst in the larger cup there is a 

 space of half an inch. The mean of several experiments 

 shows that in the small bulb the fly rotates about 50 per 

 cent, faster than in the large bulb when exposed to the 

 same source of light. 



One of the arms of another radiometer was furnished 

 with roasted mica discs blacked on alternate sides. The 

 other arm was furnished with clear mica discs. The two 

 arms were pivoted independently of each other, and one 

 of them was furnished with a minute fragment of iron, so 

 that by means of a magnet I could bring the arms in con- 

 tact, the black surface of the mica then having a clear 

 plate of mica in front of it. On bringing a lighted candle 

 near the instrument and allowing it to shine through the 

 clear plate, on the blackened mica, the clear plate is at 

 once driven away, till the arm sets at right angles to the 

 other. 



Two currents of force, acting in opposite directions, 

 can exist in the same bulb. I have prepared a double 

 radiometer in which two flys are pivoted one over the 

 other, and having their blackened sides turned in opposite 

 directions. On bringing a lighted candle near the flys 

 rotate in opposite directions. 



Experiment shows that the molecular disturbance 



