The transitional period falls between the pre 17th century alchemy and the 18th century chemistry. The climax of this period is probably with the English scientist Isaac Newton (1642 - 1727) and his book "Principia Mathematica" (1687). In this book, Newton introduced three laws of motion which served well for over two centuries in mechanical sciences. He also expanded his theories of gravitation and provided some useful explanations of the work of the Italian Scientist Galileo Galilei (1564 - 1642) who in the 1590's studied the behavior of falling bodies.
The first truly quantitative science and also links to the study of gases may be attributed to the Flemish physician, Jan Baptista Van Helmont (1577-1644) who by growing a tree under controlled conditions attempted to discover the source of living tissue in the plant. This led to his interest in air and vapor. He noted that air, vapor, breath and alcohol vapors, while appearing similar had vastly different properties. To Van Helmont, the air and vapors were without a fixed volume or shape and very much like the Greek "Chaos" from which the universe was shaped. Van Helmont named the vapors "Chaos" which sounded phonetically in Flemish is "gas"! The same word used today.
Toward the end of Van Helmont's life, gases became more and more of interest to scientists. The Italian physicists Evangelista Torriclli (1608 - 47) showed in 1643, that air exerts pressure on things. He was able to show that air could support a column of mercury thirty inches high and in doing so invented the barometer.
In 1654, Otto von Guericke (1602-86) dramatically demonstrated air pressure by inventing a pump to remove the air from within two halves of metal hemispheres and challenged teams of horsemen to separate them, unsuccessfully. When air was allowed back into the system, the spheres simply fell apart. This generated an incredible interest in the propertied of air and in particular of the Irish chemist, Robert Boyle (1627 - 91) who devised an even better air pump in an attempt to do the opposite of Guericke. Boyle tried to push air closer together and compress it. From his experiment, Boyle found that the volume of a sample of air varied with the pressure according to a simple inverse relationship. He found that by adding mercury to a sealed glass tube trapped an amount of air in one end. As he added more mercury, the air pocket became smaller. What he found was that if he doubled the amount of mercury, the volume of the air was halved. If the mercury was trippled, the volume of air was reduced to a third. As the mercury was removed, the volume increased back to its original amount. This relationship is still referred to as Boyles Law. In continental Europe, Boyle's Law is often known as Mariotte's Law after the French physicist who independently described the properties described in Boyles Law. In his publication, Mariotte was more thorough in that he specified that temperature must be constant thoughout the process. It was Boyle who revived the idea of atoms by thinking about gas as atoms with lots of space between them. This was the only way that he could explain how easily it was compressed. As gases were compressed, the empty space was simply squeezed out of them. With this view accepted, it would be easier to believe the non-compressible solid and liquids were also composed of atoms.
It is believed that Boyle coined the phrase chemist by naming his book of 1661 "The Skeptical Chymist". Skeptical because he was unwilling to blindly accept old teachings and tried to determine answers to things from basic principles and ideas. He was particularly uneasy with the old ideas about elements. He felt that all substances should be considered elements until it is proven otherwise by converting the substance into simpler substances, after which these would then be elements until proven otherwise.
By 1700 the idea of filling a vessel with steam and condensing it in such a way as to cause a moveable wall to rise and fall with each steam and condense, had been converted into the first steam engine by the English Engineer, Thomas Savery (c. 1650 - 1715). It want until Scottish Engineer, James Watt(1736 - 1819) that the safety and practicality of these steam engines were improved and began to drive machinery without human or animal power as the Industrial Revolution took-off in Britain and later in Europe.