From the 1979 lecture programme:
Atoms in solids are usually packed in a regular pattern, making crystals, which give us a hint of the existence of atoms. Molecules (which are 'family groups' of atoms) exert forces on each other when they are close, in solids and liquids. See those forces put to use in water-proofing and dishwashing.
We shall make a thin sheet of oil tell us the size of a single molecule and then the size of an atom. See the effect of water molecules bombarding a visible victim. See some strange behaviours of liquid turning to vapour: boiling and the difficulties of mountain cookery; water boiling and freezing at the same time; liquid suddenly vanishing into gas.
Atoms are far too small for us to see them, because the wavelength of the light we see by is far too large. So we should say, honestly, "you can never see atoms". Yet we make a strange promise "in the final lecture you shall see atoms". You must wait till then because you need to learn the method of the experiment and to understand the indirect interpretation. At the end of this first lecture you will see the largest (fake?) drop of water you could ever imagine.
Note: If we sketch a scale of sizes, all the way from people to atoms, on an even scale of metres or inches, the labels will be hopelessly crowded at the small-size end, with bacteria, molecules, atoms, nuclei, practically on top of each other. Instead, we spread the scale by factors, making each of the 'equal' steps one tenth of the step before.
The notes of a piano are scaled like that: as you go up in octaves, say from C to C to C... the wavelength of the sound (from crest to crest in air) goes down by a factor of 2 for each octave, from 5 metres, to 21, to 1;,-1, to 8 and so on. You will meet a similar 'logarithmic' scale in decay of radioactivity. Curiously enough human beings estimate brightness of light and loudness of sound on similar logarithmic scales.