A (Wilson) Cloud Chamber can show the tracks of ionized particles passing through it, typically emanating from outside the chamber and attributed to cosmic rays (more accurately described as cosmic particles). The chamber itself has one or more transparent surfaces, most commonly glass, through which the tracks can be observed. 1) What I do not understand is that the glass is more or less a solid substance and perhaps even a crystallized structure. How do these ionized particles, some of them quite large such as an alpha particle, pass through such barriers without interacting with the atoms or molecules therein? 2) Given a secondary particle shower, what particles so produced could pass through the chamber and which can not?