〃venomous exhalations〃 that produced diseases。 He suggested also that the air might play an important part in some processes of calcination； which； as we shall see； was proved to be true by Lavoisier late in the eighteenth century。 Boyle's notions of the exact chemical action in these phenomena were of course vague and indefinite； but he had observed that some part was played by the air； and he was right in supposing that the air 〃may have a great share in varying the salts obtainable from calcined vitriol。〃＇5＇ Although he was himself such a painstaking observer of facts； he had the fault of his age of placing too much faith in hear…say evidence of untrained observers。 Thus； from the numerous stories he heard concerning the growth of metals in previously exhausted mines； he believed that the air was responsible for producing this growthin which he undoubtedly believed。 The story of a tin…miner that； in his own time； after a lapse of only twenty…five years； a heap； of earth previously exhausted of its ore became again even more richly impregnated than before by lying exposed to the air； seems to have been believed by the philosopher。 As Boyle was an alchemist； and undoubtedly believed in the alchemic theory that metals have 〃spirits〃 and various other qualities that do not exist； it is not surprising that he was credulous in the matter of beliefs concerning peculiar phenomena exhibited by them。 Furthermore； he undoubtedly fell into the error common to 〃specialists；〃 or persons working for long periods of time on one subjectthe error of over…enthusiasm in his subject。 He had discovered so many remarkable qualities in the air that it is not surprising to find that he attributed to it many more that he could not demonstrate。 Boyle's work upon colors； although probably of less importance than his experiments and deductions upon air； show that he was in the van as far as the science of his day was concerned。 As he points out； the schools of his time generally taught that 〃color is a penetrating quality； reaching to the innermost part of the substance；〃 and； as an example of this； sealing…wax was cited； which could be broken into minute bits； each particle retaining the same color as its fellows or the original mass。 To refute this theory； and to show instances to the contrary； Boyle； among other things； shows that various colorsblue； red； yellowmay be produced upon tempered steel； and yet the metal within 〃a hair's…breadth of its surface〃 have none of these colors。 Therefore； he was led to believe that color； in opaque bodies at least； is superficial。 〃But before we descend to a more particular consideration of our subject；〃 he says； 〃 'tis proper to observe that colors may be regarded either as a quality residing in bodies to modify light after a particular manner； or else as light itself so modified as to strike upon the organs of sight； and cause the sensation we call color； and that this latter is the more proper acceptation of the word color will appear hereafter。 And indeed it is the light itself； which after a certain manner； either mixed with shades or other…wise； strikes our eyes and immediately produces that motion in the organ which gives us the color of an object。〃＇6＇ In examining smooth and rough surfaces to determine the cause of their color； he made use of the microscope； and pointed out the very obvious example of the difference in color of a rough and a polished piece of the same block of stone。 He used some striking illustrations of the effect of light and the position of the eye upon colors。 〃Thus the color of plush or velvet will appear various if you stroke part of it one way and part another； the posture of the particular threads in regard to the light； or the eye； being thereby varied。 And 'tis observable that in a field of ripe corn； blown upon by the wind； there will appear waves of a color different from that of the rest of the corn； because the wind； by depressing some of the ears more than others； causes one to reflect more light from the lateral and strawy parts than another。〃＇7＇ His work upon color； however； as upon light； was entirely overshadowed by the work of his great fellow…countryman Newton。 Boyle's work on electricity was a continuation of Gilbert's； to which he added several new facts。 He added several substances to Gilbert's list of 〃electrics；〃 experimented on smooth and rough surfaces in exciting of electricity； and made the important discovery that amber retained its attractive virtue after the friction that excited it bad ceased。 〃For the attrition having caused an intestine motion in its parts；〃 he says； 〃the heat thereby excited ought not to cease as soon as ever the rubbing is over； but to continue capable of emitting effluvia for some time afterwards； longer or shorter according to the goodness of the electric and the degree of the commotion made； all which； joined together； may sometimes make the effect considerable； and by this means； on a warm day； I； with a certain body not bigger than a pea； but very vigorously attractive； moved a steel needle； freely poised； about three minutes after I had left off rubbing it。〃＇8＇
MARIOTTE AND VON GUERICKE Working contemporaneously with Boyle； and a man whose name is usually associated with his as the propounder of the law of density of gases； was Edme Mariotte （died 1684）； a native of Burgundy。 Mariotte demonstrated that but for the resistance of the atmosphere； all bodies； whether light or heavy； dense or thin； would fall with equal rapidity； and he proved this by the well…known 〃guinea…and…feather〃 experiment。 Having exhausted the air from a long glass tube in which a guinea piece and a feather had been placed； he showed that in the vacuum thus formed they fell with equal rapidity as often as the tube was reversed。 From his various experiments as to the pressure of the atmosphere he deduced the law that the density and elasticity of the atmosphere are precisely proportional to the compressing force （the law of Boyle and Mariotte）。 He also ascertained that air existed in a state of mechanical mixture with liquids； 〃existing between their particles in a state of condensation。〃 He made many other experiments； especially on the collision of bodies； but his most important work was upon the atmosphere。 But meanwhile another contemporary of Boyle and Mariotte was interesting himself in the study of the atmosphere； and had made a wonderful invention and a most striking demonstration。 This was Otto von Guericke （1602…1686）； Burgomaster of Magdeburg； and councillor to his 〃most serene and potent Highness〃 the elector of that place。 When not engrossed with the duties of public office； he devoted his time to the study of the sciences； particularly pneumatics and electricity； both then in their infancy。 The discoveries of Galileo； Pascal； and Torricelli incited him to solve the problem of the creation of a vacuuma desideratum since before the days of Aristotle。 His first experiments were with a wooden pump and a barrel of water； but he soon found that with such porous material as wood a vacuum could not be created or maintained。 He therefore made use of a globe of copper； with pump and stop…cock； and with this he was able to pump out air almost as easily as water。 Thus； in 1650； the air…pump was invented。 Continuing his experiments upon vacuums and atmospheric pressure with his newly discovered pump； he made some startling discoveries as to the enormous pressure exerted by the air。 It was not his intention； however； to demonstrate his newly acquired knowledge by words or theories alone； nor by mere laboratory experiments； but he chose instead an open field； to which were invited Emperor Ferdinand III。； and all the princes of the Diet at Ratisbon。 When they were assembled he produced two hollow brass hemispheres about two feet in diameter； and placing their exactly fitting surfaces together； proceeded to pump out the air from their hollow interior； thus causing them to stick together firmly in a most remarkable way； apparently without anything holding them。 This of itself was strange enough； but now the worthy burgomaster produced teams of horses； and harnessing them to either side of the hemispheres； attempted to pull the adhering brasses apart。 Five； ten； fifteen teamsthirty horses； in allwere attached； but pull and tug as they would they could not separate the firmly clasped hemispheres。 The enormous pressure of the atmosphere had been most strikingly demonstrated。 But it is one thing to demonstrate； another to convince； and many of the good people of Magdeburg shook their heads over this 〃devil's contrivance；〃 and predicted that Heaven would punish the Herr Burgomaster； as indeed it had once by striking his house with lightning and injuring some of his infernal contrivances。 They predicted his future punishment； but they did not molest him； for to his fellow…citizens； who talked and laughed； drank and smoked with him； and knew him for the honest citizen that he was； he did not seem bewitched at all。 And so he lived and worked and added other facts to science； and his brass hemispheres were not destroyed by fanatical Inquisitors； but are still preserved in the royal library at Berlin。 In his experiments with his air…pump he discovered many things regarding the ac